Two-stage adiabatically coupled photonic systems

ABSTRACT

In an example, a coupled system includes a first waveguide, at least one second waveguide, and an interposer. The first waveguide has a first refractive index n1 and a tapered end. The at least one second waveguide each has a second refractive index n2. The interposer includes a third waveguide having a third refractive index n3 and a coupler portion, where n1&gt;n2&gt;n3. The tapered end of the first waveguide is adiabatically coupled to a coupler portion of one of the at least one second waveguide. A tapered end of one of the at least one second waveguide is adiabatically coupled to the coupler portion of the third waveguide of the interposer. The coupled system is configured to adiabatically couple light between the first waveguide and the at least one second waveguide and between the at least one second waveguide and the third waveguide.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of and priority to U.S.Provisional Patent Application No. 62/078,259, filed on Nov. 11, 2014,U.S. Provisional Patent Application No. 62/120,194, filed on Feb. 24,2015, U.S. Provisional Patent Application No. 62/181,679, filed on Jun.18, 2015, and U.S. Provisional Patent Application No. 62/238,542, filedon Oct. 7, 2015. The foregoing applications are incorporated herein byreference.

FIELD

The embodiments discussed herein are related to two-stage adiabaticallycoupled photonic systems.

BACKGROUND

Unless otherwise indicated herein, the materials described herein arenot prior art to the claims in the present application and are notadmitted to be prior art by inclusion in this section.

There are two common solutions to couple light into or out of a silicon(Si) photonic integrated circuit (PIC). For example, surface gratingcouplers on the Si PIC can couple light into or out of the Si PIC.However, many surface grating couplers are highly wavelength dependentand may have a relatively small pass band.

As another example, edge coupling from an edge of the Si PIC may beimplemented to couple light into or out of the Si PIC. However, the edgecoupling may require that the Si PIC have a cleaved facet and somefabs/manufacturers may be unable or unwilling to test such a process.

The subject matter claimed herein is not limited to implementations thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one example technology area where some implementationsdescribed herein may be practiced.

BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential characteristics of the claimed subject matter, nor is itintended to be used as an aid in determining the scope of the claimedsubject matter.

Some example embodiments described herein generally relate to two-stageadiabatically coupled photonic systems.

In an example embodiment, a coupled system may include a firstwaveguide, at least one second waveguide, and an interposer. The firstwaveguide may have a first refractive index n1 and a tapered end. The atleast one second waveguide may each have a second refractive index n2.The interposer may include a third waveguide having a third refractiveindex n3 and a coupler portion. The tapered end of the first waveguidemay be adiabatically coupled to a coupler portion of one of the at leastone second waveguide. A tapered end of one of the at least one secondwaveguide may be adiabatically coupled to the coupler portion of thethird waveguide of the interposer. The first refractive index n1 may begreater than the second refractive index n2, both of which may begreater than the third refractive index n3. The coupled system may beconfigured to adiabatically couple light between the first waveguide andthe at least one second waveguide and between the at least one secondwaveguide and the third waveguide.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a perspective view of an example optoelectronic system(hereinafter “system”);

FIG. 2 is a side view of an example two-stage adiabatically coupledphotonic system (hereinafter “photonic system”) of FIG. 1;

FIGS. 3A-3B include various views of portions of the photonic system ofFIGS. 1 and 2;

FIG. 4 includes a graphical representation of simulated couplingefficiency of TM polarized light from a Si waveguide to a SiN waveguideof FIGS. 3A-3B;

FIGS. 5A-5B include graphical representations of simulated light modesof TM and TE polarized light in the SiN waveguide of FIGS. 3A-3B atreference line 2;

FIG. 6 includes a graphical representation of simulated couplingefficiency of TM polarized light and TE polarized light from the SiNwaveguide to an interposer waveguide of FIGS. 3A-3B;

FIG. 7 is a side view of another example two-stage adiabatically coupledphotonic system (hereinafter “photonic system”);

FIGS. 8A-8B include various views of portions of the photonic system ofFIG. 7;

FIG. 9 is a side view of another example two-stage adiabatically coupledphotonic system (hereinafter “photonic system”);

FIG. 10 includes various simulations associated with the photonic systemof FIG. 9;

FIG. 11 is a side view of another example two-stage adiabaticallycoupled photonic system (hereinafter “photonic system”);

FIGS. 12A and 12B include an overhead view and a longitudinalcross-sectional view of another example optoelectronic system(hereinafter “system”);

FIG. 13 is an overhead view of another example optoelectronic system(hereinafter “system”);

FIG. 14 is an overhead view of an example arrayed waveguide grating(AWG) that may be formed as a passive optical device such as a WDMcomponent using SiN;

FIG. 15 is an overhead view of an example cascade of MZ interferometersthat may be formed as a passive optical device such as a WDM componentusing SiN;

FIG. 16 is a side view of another example two-stage adiabaticallycoupled photonic system (hereinafter “photonic system”);

FIG. 17 is a perspective view of an example Si PIC that defines anetched window;

FIG. 18 includes a bottom view and a side view of an implementation of aportion of an interposer that may be coupled to the Si PIC of FIG. 17within the etched window of FIG. 17;

FIGS. 19A and 19B are side views that depict alignment and attachment ofthe interposer of FIG. 18 and the Si PIC of FIG. 17;

FIG. 20 is a side view that depicts alignment of another interposer andSi PIC;

FIG. 21 is a side view that depicts alignment of another interposer andSi PIC;

FIG. 22 includes a side view and a bottom view of another arrangement ofan interposer with interposer alignment ridges and dummy interposerislands;

FIG. 23A is a side view of another example two-stage adiabaticallycoupled photonic system (hereinafter “photonic system”) that includes aSi PIC, an interposer, and an optical fiber end connector 2306(hereinafter “connector”);

FIG. 23B is a perspective view of the interposer of FIG. 23A;

FIG. 24 is a perspective view of another example photonic system(hereinafter “photonic system”);

FIGS. 25A and 25B illustrate two different offset configurations for RXvs. TX SiN waveguides;

FIG. 26 includes a side view and a bottom view of a silicon oxynitride(SiON) interposer;

FIG. 27 is a side view that depicts alignment of the SiON interposer ofFIG. 26 and the Si PIC of FIG. 17;

FIG. 28 illustrates two example optoelectronic systems (hereinafter“systems”) that each include at least one polymer on glass interposer;

FIG. 29A illustrates an example polymer on glass interposer and Si PIC;

FIG. 29B illustrates another example polymer on glass interposer;

FIG. 30 illustrates a cross-sectional view of an example Si PIC;

FIG. 31A illustrates another example Si PIC;

FIG. 31B illustrates first-third simulations for the Si PIC of FIG. 31A;

FIG. 32 illustrates a multimode SiN-to-Si adiabatic coupler region(hereinafter “coupler”);

FIGS. 33A-33D include various simulations for the coupler of FIG. 32with various different sets of parameters;

FIGS. 34A and 34B illustrate embodiments of a demultiplexer system(collectively “demultiplexer systems”);

FIG. 35 is a graphical representation of a simulation of effective indexas a function of Si waveguide width for TE and TM polarizations in Siand SiN waveguides of an adiabatic coupler region;

FIG. 36 is a graphical representation of a simulation of TE and TMpolarization coupling efficiency as a function of Si waveguide taperlength for a Si waveguide tip width of 180 nm and 150 nm;

FIG. 37 is a graphical representation of a simulation of TE and TMpolarization coupling efficiency as a function of Si waveguide taperlength for a Si waveguide tip width of 160 nm for three differentwavelength channels;

FIGS. 38A-38C illustrate example Si PIC polarization splitters orcombiners (hereinafter collectively “polarization splitters”);

FIGS. 39A and 39B include side views that depict alignment andattachment of a high index glass interposer (hereinafter “interposer”)and the Si PIC of FIG. 17;

FIG. 40A includes an upside down perspective view of another high indexglass interposer (hereinafter “interposer”); and

FIG. 40B includes a perspective view of the interposer of FIG. 40Aadiabatically coupled to a Si PIC 4008,

all arranged in accordance with at least one embodiment describedherein.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

Some embodiments described herein generally relate to adiabatic couplingof light from a silicon (Si) waveguide to an intermediate siliconnitride (Si_(x)N_(y), generically referred to herein as SiN) waveguideand then from the SiN waveguide to an interposer waveguide (e.g.,polymer or high index glass waveguide), or vice versa. For ease ofreference in the discussion that follows, the adiabatic coupling isoften discussed in the context of a single Si waveguide-to-SiNwaveguide-to-interposer waveguide coupling with the understanding thatmultiple such couplings may be included in a given system.

The Si waveguide may have a first optical mode size, the SiN waveguidemay have a second optical mode size substantially larger than the firstoptical mode size, and the polymer or other interposer waveguide mayhave a third optical mode size substantially larger than the second modesize. For example, the first optical mode size may be about 0.3 μm, orin a range between 0.25 μm and 0.5 μm; the second optical mode size maybe about 1 μm, or in a range between 0.7 μm and 3 μm; and the thirdoptical mode size may be about 10 μm, or in a range between 8 μm and 12μm. The third optical mode size may be substantially similar to anoptical mode size of a standard single mode optical fiber. For example,a standard single mode optical fiber may have an optical mode size ofabout 10 μm, which is substantially similar to the third optical modesize.

The Si waveguide may be inverse tapered to a width of about 80nanometers (nm) to increase a size of the light mode and bring it outinto a cladding of the Si waveguide. The SiN waveguide may be fabricatedon a Si photonic integrated circuit (PIC) that includes the Siwaveguide. The SiN waveguide may receive the light from the Si inversetaper. Similar to the Si waveguide, the SiN waveguide may be inversetapered to a width of 80-300 nm. The interposer waveguide withapproximately a 3-8 (μm) core may be placed in close optical contactwith the SiN waveguide. Light from the Si waveguide inverse taper may beadiabatically coupled to the SiN waveguide and then to the interposerwaveguide in steps along the direction of propagation and may becompletely or substantially completely translated to it. The interposerwaveguide may be processed on a separate rigid or flexible substrate andmay be attached to the SiN waveguide using various techniques includingthermo-mechanical attachment, or by use of index matching adhesive. TheSi PIC may include modulators, waveguides, detectors, couplers, andother optical components in a Si on Insulator (e.g., silicon on silicondioxide (SiO₂) box layer) on Si substrate. An integrated circuit (IC)may be flip chip bonded (e.g., by copper pillar) on the Si PIC in aportion of the Si PIC away from a coupling region where the SiNwaveguide and interposer waveguide may be located. The interposerwaveguide may be included in an interposer that may be transparentand/or have that may have alignment marks to allow ease in opticalalignment of the SiN waveguide on the Si PIC with the interposerwaveguide on the interposer. The interposer waveguide and the SiNwaveguide can be aligned either passively or actively.

The SiN waveguide or waveguides may be defined in a fabrication processof the Si PIC to which a SiN/SiO₂ layer section is added for couplingand passive functions. A standard Si photonic stack layer has a Sisubstrate, SiO₂ oxide layer (called BOX or SiO₂ box), and Si waveguidelayer in which Si waveguides are surrounded by SiO₂ cladding to confinethe light. Embodiments described herein may add a SiN layer to thisstandard stack for two stage coupling and optionally passive opticalfunctions. The SiN layer has regions of SiN core waveguides surroundedby SiO₂ cladding to confine the light. SiN has an intermediate index ofrefraction between indexes of refraction of Si and polymer and so allowsefficient adiabatic coupling between the two layers with taper widthsthat are within critical dimensions of some standard complementarymetal-oxide-semiconductor (CMOS) fabs. The low loss of SiN and the lowercore/cladding index difference of SiN relative to SiO₂ cladding comparedto that of Si and SiO₂ allows fabrication of passive components withbetter performance. For example wavelength division multiplexers (WDMmux) and demultiplexers (WDM demux) in SiN have higher channel isolationthan in Si. In addition, passive components in SiN have a 5× smallerdrift of peak wavelengths with temperature relative to the same in Si.

In some embodiments, transmit (TX) and receive (RX) Si waveguides on theSi PIC may be in one plane or accessible at one planar interface of theSi PIC whereas an MT connector for parallel single mode fibers can haveconfigurations by multisource agreement (MSA) in which a TX array is inone row and a RX array is in a row below it. It may also be possible forboth TX and RX to be in the same row but separated. Embodimentsdescribed herein include a an interposer that can connect from SiNwaveguide inputs/outputs in a plane of the Si PIC and present to, e.g.,an MT connector, two vertically separated rows of inputs/outputs.

In some embodiments, wavelength division multiplexing or other passiveoptical functions may be integrated in a same SiN/SiO₂ layer in whichthe SiN waveguide is formed. Use of the SiN/SiO₂ layer may beadvantageous as compared to implementing such optical functions in otherlayers and/or materials in that it may provide lower loss, betterchannel isolation due to lower loss in SiN and smaller index differencebetween core and cladding.

Some embodiments described herein may be wavelength independent over arange of operation. For instance, some embodiments described herein maybe wavelength independent over a range of operation of 1310 nm standardlong reach (LR) standards, whereas surface grating couplers may have arelatively narrow 20-30 nm pass band.

The Si waveguide and the SiN waveguide are included in different layersof the Si PIC. The Si waveguide may include Si as the waveguide coresurrounded by SiO₂ as the waveguide cladding. Similarly, the SiNwaveguide may include SiN as the waveguide core surrounded by SiO₂ asthe waveguide cladding.

In some embodiments, the layer of the Si PIC that includes the SiNwaveguide is below the layer of the Si PIC that includes the Siwaveguide and below the interposer waveguide. To make the fabrication ofthe Si/SiO₂ with SiN/SiO₂ compatible with a standard Si photonicprocess, which currently may not include a layer for the SiN waveguide,it may be possible to use wafer bonding to fabricate a structure withfully processed Si (so called Front End of Line (FEOL)) and Back End ofLine (BEOL) with SiN in a lower layer. Given this structure and a windowthat can be etched for coupling, the optical coupling between the SiNwaveguide and the interposer waveguide can be achieved. As such, lightpropagating from the Si waveguide to the SiN waveguide to the interposerwaveguide may go from the Si waveguide down to the SiN waveguide andthen up into the interposer waveguide, where it may then be coupled intoan optical fiber or the like, or light may travel on the reverse path.In these and other embodiments, the interposer waveguide can includepolymer or a high index glass waveguide having similar claddingrefractive index near 1.5.

Whether the layer of the Si PIC that includes the SiN waveguide is belowor above the layer of the Si PIC that includes the Si waveguide, the SiNwaveguide may be included in a region of the Si PIC that includes awavelength division multiplexing (WDM) component within the Si PIC.Alternatively or additionally, a SiO₂ cladding that surrounds the SiNwaveguide may be relatively thick and/or the SiN waveguide may have asquare cross-sectional profile to render the SiN waveguide polarizationinsensitive.

In some embodiments in which the layer of the Si PIC that includes theSiN waveguide is below the layer of the Si PIC that includes the Siwaveguide, a semiconductor chip with an indium phosphide (InP)-basedgain element or InP-based pin detector may be wafer bonded to the Si PICabove the layer of the Si PIC that includes the Si waveguide. In thecase of an InP-based gain element, light emitted by the InP-based gainelement may be optically coupled into the Si waveguide, and then intothe SiN waveguide, and then into the interposer waveguide, and theninto, e.g., an optical fiber. In the case of an InP-based pin detector,light received into the interposer waveguide may be coupled into the SiNwaveguide, then into the Si waveguide, and then into the InP-based pindetector.

In some embodiments, a top layer of the Si PIC may include metal‘dummies,’ at least in a region that bounds an area to be etched as anetched window for a polymer (or other material) waveguide strip thatincludes the interposer waveguide, e.g., a polymer waveguide in thisexample. Metal ‘dummies’ are arrays of metal filled holes in thedielectric stack that function to produce a mechanically flat surface onaverage over the wafer after chemical mechanical polishing (CMP) in theBEOL process. They are so called dummies because they do not function aselectrical contacts, whereas other metal in the BEOL process functionsas electrical connections between various contacts and the outputelectrical ports of the PIC. The top layer and any intervening layersdown to the layer of the Si PIC that includes the SiN waveguide may beetched through down to the layer that includes the SiN waveguide toreceive in the etched window the polymer waveguide strip and allow thepolymer waveguide to be optically coupled to the SiN waveguide. In someembodiments, polymer ridges, anchor windows, and/or dummy polymerislands may be provided to facilitate alignment and mechanicalconnection between the Si PIC and a polymer interposer that includes thepolymer waveguide.

In some embodiments, WDM components included in the Si PIC may bepolarization sensitive. For example, WDM components such as SiN basedEchelle gratings may exhibit a polarization-dependent filter function.In particular, the filter function of such WDM components may shift onepolarization of light more than another polarization of light which canlead to cross-talk for channels at a receiver. For example, a SiN basedEchelle grating may shift TE polarization at a 1310 nm wavelengthchannel to an output guide that also receives TM polarization at adifferent wavelength channel, resulting in cross-talk between the twochannels.

Accordingly, the Si PIC may additionally include a polarizationsplitter. In general, the polarization splitter may use an SiN/Siadiabatic coupler that includes two SiN waveguides and at least one Siwaveguide with two tapered ends. The tapered ends of the Si waveguidemay have tip widths that favor adiabatic coupling of one of twopolarizations of light over the other. For example, TM polarization maycouple from SiN to Si at a much narrower Si tip width than TEpolarization. The Si tip width may be selected to, in general,adiabatically couple TE polarization from the first SiN waveguidethrough the Si waveguide to the second SiN waveguide, while the TMpolarization generally remains in the first SiN waveguide.

In the discussion that follows, numerous embodiments are disclosed. Thevarious embodiments are not mutually exclusive unless context dictatesotherwise. For instance, a portion or all of one or more embodiments maybe combined with a portion or all of one or more other embodimentsunless context dictates otherwise.

Reference will now be made to the drawings to describe various aspectsof example embodiments of the invention. It is to be understood that thedrawings are diagrammatic and schematic representations of such exampleembodiments, and are not limiting of the present invention, nor are theynecessarily drawn to scale.

FIG. 1 is a perspective view of an example optoelectronic system 100(hereinafter “system 100”), arranged in accordance with at least oneembodiment described herein. As illustrated, the system 100 includes aSi PIC 102, an interposer 104, a three-dimensional (3D) stack region106, and a flip chip bonded integrated circuit (IC) 108. The Si PIC 102and the interposer 104 together form a two-stage adiabatically coupledphotonic system 200 (hereinafter “photonic system 200”).

In general, the Si PIC 104 may include one or more optical elements,such as a modulator, waveguide, coupler, or other optical element(s) ina Si-on-insulator substrate.

In general, the 3D stack region 106 may provide electrical connectionsto one or more active optical components of the Si PIC 104. Accordingly,the 3D stack region 106 may include, e.g., metallized pillars, traces,and/or contacts as well as insulative dielectric and/or other materialsand elements.

In general, the flip chip bonded IC 108 may include one or more activeand/or passive electrical devices that may be communicatively coupledthrough the 3D stack region 106 to the one or more active opticalcomponents of the Si PIC 104.

The interposer 104 may be mechanically coupled to the Si PIC 102. Aninterposer waveguide of the interposer 104 and a SiN waveguide and Siwaveguide of the Si PIC 102 may be configured to adiabatically couplelight into or out of the Si PIC 102. As used herein, light may beadiabatically coupled from one optical component or device, which herewe call the ‘initial state’ waveguide to another, here called the finalstate waveguide, in a transitional interaction region, sometimesreferred to herein as an adiabatic coupler region. To transfer opticalpower from the initial state waveguide to the final state waveguide oneor more optical properties of either or both initial and final statewaveguides, such as width, height, effective refractive index, etc. arevaried along the optical axis. Here the initial state and final statewaveguides form one system within the transitional interaction regionand light remains in a single mode of the joint system while itphysically gets transferred from the initial state waveguide to thefinal state waveguide. The initial state and final state waveguides mayrespectively correspond to the Si waveguide and the SiN waveguide, orvice versa. Alternatively or additionally, the initial and final statewaveguides may respectively correspond to the SiN waveguide and theinterposer waveguide, or vice versa. Alternatively or additionally, twocomponents may be said to be adiabatically coupled together or to eachother when the two components are configured as described herein to forman adiabatic coupler region.

Moreover, light is used generically herein to refer to electromagneticradiation of any suitable wavelength, and may include light withwavelengths of, e.g., about 800-900 nm, 2200-1360 nm, 1360-1460 nm,1530-1565 nm, or other suitable wavelengths. Light can also have TE orTM polarization.

In these and other implementations, the SiN waveguide in the Si PIC 102may be aligned with and optically coupled to the Si waveguide in the SiPIC 102. Additionally, the interposer waveguide in the interposer 104may be aligned with and optically coupled to the SiN waveguide in the SiPIC 102. The Si waveguide may have a first index of refraction n1. TheSiN waveguide may have a second index of refraction n2. The interposerwaveguide may have a third index of refraction n3. In general, thesecond index of refraction n2 of the SiN waveguide may be intermediatebetween the first index of refraction n1 of the Si waveguide and thethird index of refraction n3 of the interposer waveguide. In addition,n1>n2>n3. In some embodiments, for a two-stage adiabatically coupledphotonic system with three waveguides, each with a corresponding one ofthe indexes of refraction n1, n2, n3, the first index of refraction n1may be in a range of 3 to 3.5, the second index of refraction n2 may bein a range of 1.8 to 2.2, and the third index of refraction n3 may be ina range of 1.49 to 1.6.

The interposer waveguide in the interposer 104 may additionally bealigned with and optically coupled to an input and/or output for one ormore optical signals. An example input source may include an opticalsignal source (e.g., a laser), an optical fiber, a fiber end connector,a lens, or other optical component or device from which incoming opticalsignals (e.g., signals coming toward the Si PIC 102) are provided to theinterposer 104 for input to the Si PIC 102. An example output device towhich output may be sent may include a laser, an optical receiver (e.g.,a photodiode), an optical fiber, a fiber end connector, a lens, or otheroptical component or device to which outgoing signals (e.g., signalsleaving the Si PIC 102) may be provided through the interposer 104. Oneor more of the active optical components of the Si PIC 102 may generateor otherwise be the source of outgoing signals that are outputted fromthe photonic system 200 through the Si waveguide, the SiN waveguide, andthe interposer waveguide. Alternately or additionally, one or more ofthe active optical components of the Si PIC 102 may be configured toreceive and process incoming signals that are inputted to the photonicsystem 200 through the interposer waveguide, the SiN waveguide, and theSi waveguide.

FIG. 2 is a side view of the photonic system 200 of FIG. 1, arranged inaccordance with at least one embodiment described herein. The photonicsystem 200 includes the Si PIC 102 and the interposer 104. FIG. 2additionally illustrates the 3D stack region 106.

The Si PIC 102 includes a Si substrate 202, a SiO₂ box 204, a firstlayer 206 that includes one or more SiN waveguides 208, and a secondlayer 210 that includes one or more Si waveguides 212. In theillustrated embodiment, the first and second layer 206 and 210 are bothformed above the SiO₂ box 204. In particular, the first layer 206 isformed on (or at least above) the second layer 210 and the second layer210 is formed on (or at least above) the SiO₂ box 204. Alternatively oradditionally, a slab 214 of SiN may be formed between the first layer206 and the second layer 210 at least in a region where the Si waveguide212 is optically coupled to the SiN waveguide 208. In an exampleembodiment, the SiN waveguide 208 includes Si₃N₄ as the waveguide coresurrounded on at least two sides along its length by SiO₂ or othersuitable waveguide cladding.

Although not illustrated in FIG. 2, the Si PIC 102 may further includeone or more active optical components formed in the second layer 210. Inthese and other embodiments, the Si further include one or moredielectric layers 216 formed on and/or above the second layer 210, andone or more metallized structures 218 formed in the dielectric layers216. The metallized structures 218 may extend from a top of the Si PIC102 through the dielectric layers 216 to electrical contact with theactive optical components formed in the second layer 210 or elsewhere inthe Si PIC 102. The dielectric layers 216 may include SiO₂ or othersuitable dielectric material. The dielectric layers 216 and themetallized structures 218 are collectively an example of the 3D stackregion 106.

With combined reference to FIGS. 1 and 2, the flip chip bonded IC 108may be flip chip bonded to the 3D stack region 106. The flip chip bondedIC may include one or more active and/or passive electrical devices thatmay be communicatively coupled through the 3D stack region 123 to theone or more active optical components formed in the second layer 210 ofthe Si PIC 102.

The interposer 104 may include an interposer substrate 220 and awaveguide strip 222 formed on and/or coupled to the interposer substrate220. The waveguide strip 222 includes one or more interposer waveguides224. Each interposer waveguide 224 includes an interposer core 224A andan interposer cladding 224B of different indexes of refraction. Acoupler portion of the interposer waveguide 224 may be disposed above atapered end of the SiN waveguide 208 in the first layer 206 and isaligned with the tapered end of the SiN waveguide 208 as described inmore detail below.

The Si waveguide 212 (or more particularly, the core of the Si waveguide212) may have the first index of refraction n₁ mentioned above. The SiNwaveguide 208 (or more particularly, the core of the SiN waveguide 208)may have the second index of refraction n₂ mentioned above. Theinterposer waveguide 224 (or more particularly, the interposer core 224Aof the interposer waveguide 224) may have the third index of refractionn₃ mentioned above, where n₁>n₂>n₃.

FIGS. 3A-3B include various views of portions of the photonic system 200of FIG. 2, arranged in accordance with at least one embodiment describedherein. In particular, FIG. 3A includes an overhead view 300A and alongitudinal cross-sectional view 300B and FIG. 3B includes transversecross-sectional views 300C-300F at locations respectively denoted byreference lines 1-4 in FIG. 3A.

The overhead view 300A of FIG. 3A illustrates relative x-axis and z-axisalignment of various components with respect to each other according toan arbitrarily defined x-y-z coordinate axis provided within each of theviews 300A-300B of FIG. 3A and provided in other Figures herein. Asingle instance of the x-y-z coordinate axis is provided for all fourviews 300C-300F of FIG. 3B since all four views 300C-300F have the sameorientation. The x direction may sometimes be referred to as a lateralor transverse direction and terms such as width, lateral, transverse,side, sideways etc. may be used to refer to, e.g., dimensions, relativeposition, and/or movement in the x direction unless context dictatesotherwise. The y direction may sometimes be referred to as a verticaldirection and terms such as height, thickness, vertical, vertically,above, below, up, down, etc. may be used to refer to, e.g., dimensions,relative position, and/or movement in the y direction unless contextdictates otherwise. The z direction may sometimes be referred to as alongitudinal or light-propagating direction and terms such as length,longitudinal, upstream, downstream, forward, backward, front, back, etc.may be used to refer to, e.g., dimensions, relative position, and/ormovement in the z direction unless context dictates otherwise.

The longitudinal cross-sectional view 300B of FIG. 3A illustrates anexample material stack up for the various components. The overhead view300A of FIG. 3A includes outlines or footprints of the variouscomponents at different levels in the material stack up that may notnecessarily be visible when viewed from above, but are shown as outlinesor footprints to illustrate the x and z alignment of the variouscomponents with respect to each other.

The portion of the photonic system 200 illustrated in the view 300A ofFIG. 3A includes a tapered end of the Si waveguide 212. The tapered endof the Si waveguide 212 is relatively wider at reference line 1 than atreference line 2. The tapered end of the Si waveguide 212 may beconsidered to have a taper or an inverse taper, which are structurallyequivalent. As used herein, a waveguide such as the Si waveguide 212 ofFIG. 3A may be considered to have a taper with respect to incomingoptical signals, e.g., optical signals that enter the waveguide at arelatively narrower portion of the waveguide and propagate through thewaveguide towards a relatively wider portion of the waveguide. Incomparison, a waveguide such as the Si waveguide 212 of FIG. 3A may beconsidered to have an inverse taper with respect to outgoing opticalsignals, e.g., optical signals that propagate through the waveguide inthe direction from wider to narrower to exit the waveguide. Forsimplicity in the discussion that follows, the term “taper” and itsvariants should be broadly construed as a variation of the waveguidewidth along the optical axis. In some embodiments, it may beadvantageous to vary the width of the waveguide along the optical axislinearly or nonlinearly or in segments of linear and nonlinearvariation. The width of the taper around the interaction region of theinitial state and final state waveguides may be varied to optimizecoupling or reduce the length of the coupling region to produce aphysically smaller device.

The Si waveguide 212, including the tapered end, may be formed in thesecond layer 210 and positioned below the first layer 206 that includesthe SiN waveguide 208. For example, the second layer 210 may bepositioned below the SiN slab 214, which in turn is positioned below thefirst layer 206. Within the second layer 210, SiO₂ may generally bedisposed adjacent to sides of the Si waveguide 212 (e.g., in thepositive x and negative x directions), as illustrated in the views 300Cand 300D of FIG. 3B, to form a cladding for the Si waveguide 212, whichserves as the core. In some embodiments, the Si waveguide 212 and/orother Si waveguides of the Si PIC 102 may have a thickness t_(Si) (e.g.,in the y direction) of approximately 0.3 μm and an index of refractionof about 3.4. The specific values of indexes of refraction, thickness,width, length, and other values provided herein are provided by way ofexample only and values other than those explicitly stated maynevertheless fall within the scope of the described embodiments.

As illustrated in FIG. 3A, the SiN slab 214 may be formed or otherwiselocated on the second layer 210 that includes the Si waveguide 212. TheSiN slab 214 may have a thickness (e.g., in the y direction) ofapproximately 0-50 nm in some embodiments.

The view 300B of FIG. 3A further illustrates the SiN waveguide 208. TheSiN waveguide 208 includes both a coupler portion and a tapered end. Thecoupler portion of the SiN waveguide 208 generally includes the portionof the SiN waveguide 208 between reference lines 1 and 2 and the taperedend of the SiN waveguide 208 generally includes the portion of the SiNwaveguide 208 between reference lines 3 and 4. The tapered end of theSiN waveguide 208 is relatively wider at reference line 3 than atreference line 4. Within the first layer 206, SiO₂ may generally bedisposed adjacent to sides of the SiN waveguide 208 (e.g., in thepositive x and negative x directions), to serve as a cladding layer forthe SiN waveguide 208, as illustrated in the views 300C-300F of FIG. 3B.In some embodiments, the SiN waveguide 208 and/or other SiN waveguidesof the first layer 206 may have a thickness (e.g., in the z direction)of approximately 0.5-1 μm and an index of refraction of about 1.99.

It can be seen from FIG. 3A that, although the SiN waveguide 208 isdisplaced in the y direction from the Si waveguide 212, the tapered endof the Si waveguide 212 is aligned in the x and z directions with thecoupler portion of the SiN waveguide 208 such that the tapered end ofthe Si waveguide 212 overlaps the coupler portion of the SiN waveguide208 (as seen in the view 300A) in the x and z directions and is parallelthereto (as seen in the view 300B).

FIG. 3A additionally illustrates the interposer waveguide 224. Theinterposer waveguide 224 includes the core 224A and cladding 224B.Additionally, the interposer waveguide 224 includes both a couplerportion and an end that extends from the coupler portion. The couplerportion of the interposer waveguide 224 generally includes the portionof the interposer waveguide 224 between reference lines 3 and 4 and theend extends away from the coupler portion (e.g., to the right in FIG.3A). The interposer waveguide 224 may be coupled, along with potentiallyone or more other interposer waveguides, to the interposer substrate 220of FIG. 2. In some embodiments, the interposer waveguide 224 and/orother interposer waveguides of the interposer 104 of FIG. 2 may have athickness ti (e.g., in the y direction) of approximately 3 μm, a widthw₁ (e.g., in the x direction) of about 4 μm, and an index of refractionof about 1.51 for the interposer core 224A and about 1.5 for theinterposer cladding 224B. More generally, provided the index ofrefraction of the interposer core 224A is greater than that of theinterposer cladding 224B, the interposer core 224A may have an index ofrefraction in a range from 1.509 to 1.52. Note that the low end of therange of refractive index for the interposer is determined by theminimum taper tip width afforded by the SiN fabrication process, whichhere is assumed to be on the order of 200 nm. For instance, the minimumtaper tip width for SiN waveguides may be 180 nm. If the process allowsfor a smaller tip width for the SiN, a correspondingly lower refractiveindex for the interposer will be allowed. This is because adiabaticcoupling transition occurs when the effective indices of the SiNwaveguide and interposer waveguide are substantially the same.Decreasing the SiN tip width (by using a more sophisticated process, forexample) reduces the effective index of the SiN waveguide allowing alower material index for the interposer.

It can be seen from FIG. 3A that, although the interposer waveguide 224is displaced in the y direction from the SiN waveguide 208, the couplerportion of the interposer waveguide 224 is nevertheless aligned in the xand z directions with the tapered end of the SiN waveguide 208 such thatthe coupler portion of the interposer waveguide 224 overlaps the taperedend of the SiN waveguide 208 (as seen in the view 300A) and is parallelthereto (as seen in the view 300B).

The views 300C-300F of FIG. 3B depict widths (e.g., in the x direction)of the tapered end of each of the Si waveguide 212 and the SiN waveguide208 at, respectively, reference lines 1-4 of FIG. 3A. For instance, fromthe views 300C and 300D, it can be seen that a width of the Si waveguide212 tapers from a width w_(Si1) of about 0.32 μm at reference line 1 toa width w_(Si2) of about 0.08 μm (or 80 nm) at reference line 2. Also,from the views 300E and 300F, it can be seen that a width of the SiNwaveguide 208 tapers from width w_(SiN1) of about 1.0 μm at referenceline 3 to width w_(SiN2) of about 0.20 μm (or 200 nm) at reference line4. As another design example, the width w_(SiN1) can be about 1.5 μm atreference line 3 tapered to the width w_(SiN2) of about 0.08 μm atreference line 4.

The tapered ends of the Si waveguide 212 and the SiN waveguide 208provide adiabatic transitions for optical signals from the Si waveguide212 to the SiN waveguide 208 and from the SiN waveguide 208 to theinterposer waveguide 224, or adiabatic transitions for optical signalstraveling in the opposite direction. An adiabatic transition may beachieved by changing the structure and/or an effective index of thetapered ends of the Si and SiN waveguides 212 and 208 in a sufficientlyslow manner so light is not scattered from its mode when it is incidenton the tapered ends and continues propagating in this same mode when itexits the tapered ends and enters the coupler portion of the SiNwaveguide 208 or the interposer waveguide 224. That is, the light mayexperience a gradual transition between the tapered end of the Si or SiNwaveguide 212 or 208 and the y-axis displaced and adjacent couplerportion of the SiN or interposer waveguide 208 or 224 such that the modedoes not change and no significant scattering of light takes place.Accordingly, the tapered end of the Si waveguide 212 combined with thecoupler portion of the SiN waveguide 208 is an example of an adiabaticcoupler region. The tapered end of the SiN waveguide 208 and the couplerportion of the interposer waveguide 224 is another example of anadiabatic coupler region.

In operation, the structure, refractive index, and/or othercharacteristics of an optical medium may determine an effective index ofthe optical medium. Effective index is somewhat analogous to energylevels in quantum mechanics. Higher effective index is analogous tolower energy level. Thus, for two adjacent optical media with differenteffective indexes, light tends to propagate through the medium with thehigher effective index.

In the embodiments described herein, and with particular reference toFIGS. 3A and 3B, Si waveguides may generally have a higher effectiveindex than SiN waveguides, and SiN waveguides may generally have ahigher effective index than polymer waveguides. By tapering the end of aSi waveguide, the effective index may be reduced along the length of thetapered end until the effective index of the Si waveguide approximatelymatches or even becomes smaller than the effective index of a y-axisdisplaced SiN waveguide, such as illustrated in FIGS. 3A and 3B.Accordingly, light propagating through the Si waveguide 212 and exitingthrough its tapered end may exit the tapered end of the Si waveguide 212and enter the SiN waveguide 208 about at a point where the effectiveindex of the tapered end of the Si waveguide 212 matches an effectiveindex of the SiN waveguide 208. Analogously, the SiN waveguide 208 maybe tapered at its end until its effective index approximately matches oreven becomes smaller than the effective index of a y-axis displacedpolymer waveguide, such as illustrated in FIGS. 3A and 3B. Accordingly,light propagating through the SiN waveguide 208 and exiting through itstapered end may exit the tapered end of the SiN waveguide 208 and enterthe interposer waveguide 224 about at a point where the effective indexof the tapered end of the SiN waveguide 208 matches an effective indexof the interposer waveguide 224.

Some other adiabatic coupling systems include a single adiabatic couplerregion or stage in which a polymer or high index glass (or otherinterposer) waveguide receives light directly from a tapered end of a Siwaveguide. Such systems generally require a Si waveguide that is verythin (e.g., 190-200 nm thick in the y direction of FIGS. 3A-3B) and/ortapering the Si waveguide to a very thin width (e.g., 40 nm wide in thex direction) to reach an effective index small enough to match theeffective index of the polymer or high index glass waveguide. Such finedimensions may not be achievable for some fabs/manufacturers and/or maybe inconsistent with existing processes of these fabs/manufacturers. Inaddition, smaller Si waveguides generally have higher insertion lossthan relatively larger Si waveguides, making them disadvantageous. Theadiabatic coupling length between Si and Polymer waveguides may be onthe order of 2 mm, over which such a narrow Si waveguide would introduceunwanted optical loss. In comparison, some embodiments described hereinimplement a two-stage adiabatic coupling where the SiN waveguide has anintermediate index of refraction between that of the Si waveguide and ofthe interposer waveguide, such that the effective index of the Siwaveguide may be matched to the effective index of the SiN waveguide byfabricating the SiN waveguide and/or its tapered end with largerdimensions that are achievable by the fabs/manufactures and that allowthe use of a larger, lower loss SiN waveguide. Here, the adiabaticcoupling length from the Si waveguide to the SiN waveguide may be quitesmall, e.g., about 50-200 μm. In this case the higher loss of the small80 nm wide Si waveguide does not introduce significant loss and the lossis significantly less than the narrower Si waveguide over 2 mm asdescribed above. The adiabatic coupler region between the SiN waveguideand the interposer waveguide may be around 2 mm, where the lower loss ofthe SiN waveguide relative to the Si waveguide leads to less loss ascompared with direct adiabatic coupling between Si and interposerwaveguides.

FIG. 4 includes a graphical representation of simulated couplingefficiency of TM polarized light from the Si waveguide 212 to the SiNwaveguide 208 of FIGS. 3A-3B, arranged in accordance with at least oneembodiment described herein. The horizontal axis of FIG. 4 is height orthickness t_(SiN) (e.g., in the y direction of FIGS. 3A-3B) of the SiNwaveguide 208 and the vertical axis is the coupling efficiency. It canbe seen from FIG. 4 that the coupling efficiency increases withincreasing height or thickness t_(SiN) of the SiN waveguide 208. At aheight or thickness t_(SiN) of 1 μm, the coupling efficiency isapproximately 96% for the TM polarized light.

FIGS. 5A-5B include graphical representations of simulated light modesof TM and TE polarized light in the SiN waveguide 208 of FIGS. 3A-3B atreference line 2, arranged in accordance with at least one embodimentdescribed herein. For the simulations of FIGS. 5A-5B, the SiN waveguide208 is assumed to have a height or thickness t_(SiN) (e.g., in the ydirection) of about 1 μm and a width w_(SiN1) (e.g., in the x direction)of about 1.5 μm.

As illustrated in FIG. 5A, at reference line 2 in FIGS. 3A-3B, most ofthe TM polarized light has moved into the SiN waveguide 208, althoughsome still remains in the tip of the tapered end of the Si waveguide212. As illustrated in FIG. 5B, at reference line 2 in FIGS. 3A-3B,virtually all of the TE polarized light has moved out of the Siwaveguide 212 and into the SiN waveguide 208.

FIGS. 5A-5B further illustrate the light as a single mode of light.However, SiN waveguides 208 may in some cases support multimode light.When single mode light is coupled adiabatically from the Si waveguide212 to the SiN waveguide 208, only the single mode of the SiN waveguide208 may be excited and the light may stay in the single mode in someembodiments. In other embodiments, a Si—SiN adiabatic coupler region maybe configured to support transmission therebetween multimodes of light,as discussed below. In other embodiments, the SiN waveguide may beconfigured to support only the single mode.

FIG. 6 includes a graphical representation of simulated couplingefficiency of TM polarized light and TE polarized light (respectivelylabeled “TM” and “TE” in FIG. 6) from the SiN waveguide 208 to theinterposer waveguide 224 of FIGS. 3A-3B, arranged in accordance with atleast one embodiment described herein. The horizontal axis of FIG. 6 islength (e.g., in the z direction of FIGS. 3A-3B) of the tapered end ofthe SiN waveguide 208 and the vertical axis is the coupling efficiency.It can be seen from FIG. 6 that the coupling efficiency is generallybetter for TE polarized light and increases for both TE and TM polarizedlight with increasing length of the tapered end of the SiN waveguide208.

FIG. 7 is a side view of another example two-stage adiabatically coupledphotonic system 700 (hereinafter “photonic system 700”), arranged inaccordance with at least one embodiment described herein. The photonicsystem 700 includes a Si PIC 702 and an interposer 704. Similar to thephotonic system 200, the photonic system 700 may generally be configuredto adiabatically couple light into and/or out of the photonic system700.

The Si PIC 702 includes a Si substrate 706, a SiO₂ box 708, a firstlayer 710 that includes a SiN waveguide 712, and a second layer 714 thatincludes a Si waveguide 716. In the illustrated embodiment, the firstlayer 710 is formed on (or at least above) the SiO₂ box 708 and thesecond layer 714 is formed on (or at least above) the first layer 710.Alternatively or additionally, a slab 718 of SiN may be formed betweenthe first layer 710 and the second layer 714 at least in a region wherethe Si waveguide 716 is optically coupled to the SiN waveguide 712. Inan example embodiment, the SiN waveguide 712 includes Si₃N₄ as thewaveguide core surrounded on at least two sides along its length by SiO₂or other suitable waveguide cladding.

As illustrated in FIG. 7, the Si PIC 702 may further include one or moreactive optical components 720 formed in the second layer 714, one ormore dielectric layers 722 formed on and/or above the second layer 714,and one or more metallized structures 724 formed in the dielectriclayers 722. The metallized structures 724 may extend from a top of theSi PIC 702 through the dielectric layers 722 to electrical contact withthe active optical components 720. The dielectric layers 722 may includeSiO₂ or other suitable dielectric material. The dielectric layers 722and the metallized structures 724 are collectively an example of a 3Dstack region that may be included in Si PICs, such as the Si PIC 702 ofFIG. 7. Alternatively or additionally, the region of the Si PIC 702 thatincludes the active optical components 720 may be referred to as anactive region of the Si PIC 702 (labeled “Actives” in FIG. 7), whereas aregion or regions of the Si PIC 702 that lack such active opticalcomponents 720 may be referred to as a passive region of the Si PIC 702(labeled “Passives” in FIG. 7).

The Si PIC 702 may define an etched window 725 through the layers of theSi PIC 702 down to the first layer 710, including through the dielectriclayers 722, the second layer 714, and the SiN slab 718 in the example ofFIG. 7.

The interposer 704 may include an interposer substrate 726 and awaveguide strip 728 formed on and/or coupled to the polymer substrate.The waveguide strip 728 includes one or more interposer waveguides 730.Each of the interposer waveguides 730 includes an interposer core andinterposer cladding of different indices of refraction. A couplerportion of each interposer waveguide 730 is disposed above a tapered endof each SiN waveguide 712 within the etched window 725 of the Si PIC 702and is aligned with the tapered end of the corresponding SiN waveguide712 as described in more detail below.

Each of the Si PIC 702, the interposer 704, the Si substrate 706, theSiO₂ box 708, the first layer 710, the SiN waveguide 712, the secondlayer 714, the Si waveguide 716, the SiN slab 718, the active opticalcomponents 720, the dielectric layers 722, the metallized structures724, the interposer substrate 726, the waveguide strip 728, and theinterposer waveguide 730 of FIG. 7 may generally be similar or identicalto, respectively, any of the other Si PICs, interposers, Si substrates,SiO₂ boxes, first layers, SiN waveguides, second layers, Si waveguides,SiN slabs, active optical components, dielectric layers, metallizedstructures, interposer substrates, waveguide strips, and interposerwaveguides disclosed herein, excepted as otherwise indicated herein.

FIGS. 8A-8B include various views of portions of the photonic system 700of FIG. 7, arranged in accordance with at least one embodiment describedherein. In particular, FIG. 8A includes an overhead view 800A and alongitudinal cross-sectional view 800B and FIG. 8B includes transversecross-sectional views 800C-800F at locations respectively denoted byreference lines 1-4 in FIG. 8A.

The overhead view 800A of FIG. 8A illustrates relative x-axis and z-axisalignment of various components with respect to each other. Thelongitudinal cross-sectional view 800B of FIG. 8A illustrates an examplematerial stackup for the various components. The overhead view 800A ofFIG. 8A includes outlines or footprints of the various components atdifferent levels in the material stackup that may not necessarily bevisible when viewed from above, but are shown as outlines or footprintsto illustrate the x and z alignment of the various components withrespect to each other.

The portion of the photonic system 700 illustrated in the view 800A ofFIG. 8A includes a tapered end of the Si waveguide 716. The tapered endof the Si waveguide 716 is relatively wider at reference line 1 than atreference line 2. The Si waveguide 716, including the tapered end, maybe formed in the second layer 714 (FIG. 7) on or above the first layer710 (FIG. 7) that includes the SiN waveguide 712. For example, thesecond layer 714 may be formed on the SiN slab 718 above the first layer710. Within the second layer 714, SiO₂ may generally be disposedadjacent to sides of the Si waveguide 716 (e.g., in the positive x andnegative x directions), as illustrated in the views 800C and 800D ofFIG. 8B, to form a cladding for the Si waveguide 716, which serves asthe core. The thickness and/or index of refraction of the Si waveguide716 may be the same as or different than the thickness and/or index ofrefraction of the Si waveguide 212 described above.

As illustrated in FIG. 8A, the SiN slab 718 may be formed or otherwiselocated on the first layer 710 (FIG. 7) that includes the SiN waveguide712. The SiN slab 718 may have a thickness that is the same as ordifferent than the thickness of the SiN slab 214 described above.

The view 800B of FIG. 8A further illustrates the SiN waveguide 712. TheSiN waveguide 712 includes both a coupler portion and a tapered end. Thecoupler portion of the SiN waveguide 712 generally includes the portionof the SiN waveguide 712 between reference lines 1 and 2 and the taperedend of the SiN waveguide 712 generally includes the portion of the SiNwaveguide 712 between reference lines 3 and 4. The tapered end of theSiN waveguide 712 is relatively wider at reference line 3 than atreference line 4. Within the first layer 710 (FIG. 7), SiO₂ maygenerally be disposed adjacent to sides of the SiN waveguide 712, toserve as a cladding layer for the SiN waveguide 712 (e.g., in thepositive x and negative x directions), as illustrated in the views800C-800F of FIG. 8B. The SiN waveguide 712 and/or other SiN waveguidesof the first layer 710 may have a thickness (e.g., in the y direction)and/or index of refraction that is the same as or different than thethickness and/or index of refraction of the SiN waveguide 208 describedabove.

It can be seen from FIG. 8A that, although the SiN waveguide 712 isdisplaced in the y direction from the Si waveguide 716, the tapered endof the Si waveguide 716 is aligned in the x and z directions with thecoupler portion of the SiN waveguide 712 such that the tapered end ofthe Si waveguide 716 overlaps the coupler portion of the SiN waveguide712 (as seen in the view 800A) in the x and z directions and is parallelthereto (as seen in the view 800B).

FIG. 8A additionally illustrates the interposer waveguide 730. Theinterposer waveguide 730 includes an interposer core 730A and aninterposer cladding 730B. Additionally, the interposer waveguide 730includes both a coupler portion and an end that extends from the couplerportion. The coupler portion of the interposer waveguide 730 generallyincludes the portion of the interposer waveguide 730 between referencelines 3 and 4 and the end extends away from the coupler portion (e.g.,to the right in FIG. 8A). The interposer waveguide 730 may be coupled,along with potentially one or more other interposer waveguides, to theinterposer substrate 726 of FIG. 7. In some embodiments, the interposerwaveguide 730 and/or other interposer waveguides of the interposer 704of FIG. 7 may have a thickness (e.g., in the y direction), a width(e.g., in the x direction) and/or an index of refraction that is thesame as or different than the thickness, width, and/or index ofrefraction of the interposer waveguide 224 described above.

It can be seen from FIG. 8A that, although the interposer waveguide 730is displaced in the y direction from the SiN waveguide 712, the couplerportion of the interposer waveguide 730 is nevertheless aligned in the xand z directions with the tapered end of the SiN waveguide 712 such thatthe coupler portion of the interposer waveguide 730 overlaps the taperedend of the SiN waveguide 712 (as seen in the view 800A) and is parallelthereto (as seen in the view 800B).

The Si waveguide 716, the SiN waveguide 712, tapered ends thereof,and/or the interposer waveguide 730 may have widths (e.g., in the xdirection) and/or lengths (e.g., in the z direction) that are the sameas or different than the widths and/or lengths of the Si waveguide 212,the SiN waveguide 208, tapered ends thereof, and/or the interposerwaveguide 224 described above. Alternatively or additionally, thetapered ends of the Si waveguide 716 and the SiN waveguide 712 mayprovide adiabatic transitions for optical signals from the Si waveguide716 to the SiN waveguide 712 and from the SiN waveguide 712 to theinterposer waveguide 730, as described above with respect to the Siwaveguide 212, the SiN waveguide 208, and the interposer waveguide 224.

FIG. 9 is a side view of another example two-stage adiabatically coupledphotonic system 900 (hereinafter “photonic system 900”), arranged inaccordance with at least one embodiment described herein. The photonicsystem 900 is similar in many respects to the photonic system 700discussed above, and includes a Si PIC 902 and the interposer 704. TheSi PIC 902 is similar in many respects to the Si PIC 702 discussedabove, and includes, for example, the SiO₂ box 708, the second layer714, the Si waveguide 716, the active optical components 720, thedielectric layers 722, and the metallized structures 724, and the Si PIC902 additionally defines an etched window 925.

The Si PIC 902 additionally includes a first layer 910 that is similarto the first layer 710 of FIG. 7. In particular, the first layer 910includes a first SiN waveguide 912A with a coupler portion that issimilar to the SiN waveguide 712 with coupler portion discussed above.In particular, the tapered end of the Si waveguide 716 and the couplerportion of the first SiN waveguide 912A are aligned with each other asdescribed with respect to the Si waveguide 716 and the SiN waveguide 712so as to adiabatically couple light from the Si waveguide 716 to thefirst SiN waveguide 912A, or vice versa.

The first layer 910 of the Si PIC 902 additionally includes a WDMcomponent, generally designated at 914. The WDM component 914 mayfunction as a WDM mux or WDM demux, for instance. The WDM component 914may include one or more cascaded Mach-Zehnders, Echelle gratings, orarrayed waveguide gratings (AWGs). The WDM component 914 opticallycouples the first SiN waveguide 912A to one or more second SiNwaveguides 912B, 912C according to the wavelength of light.Alternatively or additionally, the WDM component 914 may opticallycouple one or each of the second SiN waveguides 912B, 912C that can becarrying optical signals having different wavelength to one or morefirst SiN waveguides 912A that are in turn coupled to one or more Siwaveguides 716. The second SiN waveguide 912C may include a tapered endto adiabatically couple light into the interposer waveguide 730, asdescribed with respect to the SiN waveguide 712 and the interposerwaveguide 730 above.

To reduce and/or eliminate polarization dependence of the WDM component914, one or more of the first and second SiN waveguides 912A-912C(generically hereafter “SiN waveguide 912” or “SiN waveguides 912”) mayhave the same effective index and group index for TE and TMpolarizations of light. To configure the SiN waveguide 912 with the sameeffective index and group index for TE and TM polarizations of light,the SiN waveguide 912 may be provided with a symmetric squarecross-section and may generally be surrounded by SiO₂.

For example, in FIG. 9, at least the SiN waveguide 912B may have asquare cross-section along its length, or along at least a portionthereof. The square cross-section along at least a portion of the lengthof the SiN waveguide 912B may be about 500 nm by about 500 nm.Laterally, the SiN waveguide 912B may have SiO₂ adjacent thereto. In thevertical direction (e.g., the y direction), the SiN waveguide 912B mayhave the SiO₂ box 708 or another layer of SiO₂ beneath and adjacentthereto, where the SiO₂ box 708 or other layer of SiO₂ has a thicknessof at least 200 nm. Further, the SiN waveguide 912B may have one or morelayers of SiO₂ above and adjacent thereto, such as the second layer 714and/or the dielectric layers 722. The one or more layers of SiO₂ thatare above and adjacent to the SiN waveguide 912B in FIG. 9 may have anaggregate thickness greater than 330 nm.

FIG. 10 includes various simulations 1000A-1000C associated with theembodiment of FIG. 9, arranged in accordance with at least oneembodiment described herein. The simulation 1000C depicts effectiveindex/group index of the SiN waveguide 912B of FIG. 9 as a function ofwidth of the SiN waveguide 912B where it is assumed that the SiNwaveguide 912B has a thickness of 500 nm. In the simulation 1000C,curves 1002A and 1002B represent the group index of the SiN waveguide912B for, respectively, the TE and TM polarizations of light, whilecurves 1004A and 1004B represent the effective index of the SiNwaveguide 912B for, respectively, the TE and TM polarizations of light.It can be seen from the simulation 1000C that the same group index andeffective index for the TE and TM polarizations of light occurs at 500nm, e.g., where the width of the SiN waveguide 912B is equal to the 500nm thickness. This may result in zero birefringence operation.

FIG. 10 additionally includes a table 1006 that lists the 500 nm by 500nm cross-sectional measurement of the SiN waveguide 912B determined fromthe simulation 1000C, as well as indexes of refraction of the SiN andSiO₂ used in the SiN waveguide.

The simulations 1000A and 1000B of FIG. 10 assume the parameters listedin the table 1004. It can be seen from the simulations 1000A and 1000Bthat zero birefringence operation occurs for the 500 nm by 500 nm SiNwaveguide 912B surrounded by SiO₂ on all four sides along the length ofthe SiN waveguide 912B.

FIG. 11 is a side view of another example two-stage adiabaticallycoupled photonic system 1100 (hereinafter “photonic system 1100”),arranged in accordance with at least one embodiment described herein.The photonic system 1100 is similar in many respects to the photonicsystem 900 discussed above and includes, inter alia, the interposer 704and an Si PIC 1102 with the SiO₂ box 708, a first layer 1110 thatincludes one or more SiN waveguides 1112A-1112C (hereinafter “SiNwaveguide 1112” or “SiN waveguides 1112”) and a WDM component 1113, asecond layer 1114 that includes one or more Si waveguides 1116, one ormore dielectric layers 1122, and metallization structures 1124. Thefirst layer 1110, SiN waveguide 1112, WDM component 1113, second layer1114. Si waveguide 1116, dielectric layers 1122, and metallizationstructures 1124 may generally be similar or identical to, respectively,any of the other first layers, SiN waveguides, WDM components, secondlayers. Si waveguides, dielectric layers, and metallization structuresdisclosed herein except as otherwise indicated herein.

One difference between the photonic system 1100 and, e.g., the photonicsystem 900 is that the first and second layers 1110 and 1114 of the SiPIC 1102 of FIG. 11 are switched compared to the first and second layers910 and 714 of the Si PIC 902 of FIG. 9. In particular, in FIG. 11, thesecond layer 1114 that includes the Si waveguide 1116 is below the firstlayer 1110 that includes the SiN waveguides 1112. The dielectric layers1122 may be disposed above and in contact with the first layer 1110 andmay have a thickness greater than 800 nm. The second layer 1114 may bedisposed beneath and in contact with the first layer 1110 and may have athickness greater than 330 nm.

The Si PIC 1102 may otherwise generally be similar to the Si PIC 902 ofFIG. 9. For example, light may be adiabatically coupled from the Siwaveguide 1116 to the SiN waveguide 1112A, or vice versa, and from theSiN waveguide 1112C to the interposer waveguide 730, or vice versa, in asimilar manner as described above. Additionally, the SiN waveguide 1112Bmay have the same effective index and group index for TE and TMpolarizations of light.

FIGS. 12A and 12B include an overhead view and a longitudinalcross-sectional view of another example optoelectronic system 1200(hereinafter “system 1200”) that includes two adiabatic coupler regionsmade up of the Si waveguide 212, the SiN waveguide 208, and theinterposer waveguide 224 of FIGS. 3A-3B, arranged in accordance with atleast one embodiment described herein.

The system 1200 further includes a distributed feedback (DFB) laser 1202or other semiconductor laser, a first lens 1204, an optical isolator1206, and a second lens 1208, all mounted to a laser sub-mount 1210. Thefirst lens 1204 may be positioned in an optical path of an opticalsignal output from the DFB laser 1202. The optical isolator 1206 may bepositioned in the optical path after the first lens 1204. The secondlens 1208 may be positioned in the optical path after the opticalisolator 1206. As illustrated, an end of the interposer waveguide 224may be positioned in the optical path after the second lens 1208.

FIG. 13 is an overhead view of another example optoelectronic system1300 (hereinafter “system 1300”), arranged in accordance with at leastsome embodiments described herein. The system 1300 includes N (N>=2) DFBlasers 1302A-1302D configured to emit optical signals of differentwavelengths λ1-λN, where N is 4 in the example of FIG. 13. Each of theDFB lasers 1302A-1302D is optically coupled to a correspondinginterposer waveguide 1304A-1304D through a corresponding first lens1306A-1306D, a corresponding optical isolator 1308A-1308D, and acorresponding second lens 1310A-1310D as described with respect to FIGS.12A-12B.

The output of each of the DFB lasers 1302A-1302D is received by acorresponding one of the interposer waveguides 1304A-1304D (each made upof an interposer core and interposer cladding and formed on aninterposer substrate) and is adiabatically coupled from thecorresponding interposer waveguide 1304A-1304D into a corresponding SiNwaveguide 1312A-1312D included in a first layer of a Si PIC included inthe system 1300 of FIG. 13. The Si PIC of the system 1300 may be similaror identical to one or more of the other Si PICs described herein. Theadiabatic coupling is accomplished as described above, e.g., byproviding the SiN waveguides 1312A-1312D with tapered ends that arealigned in two orthogonal dimensions with a corresponding couplerportion of the corresponding interposer waveguides 1304A-1304D. Ratherthan each of the SiN waveguides 1312A-1312D adiabatically coupling acorresponding one of N optical signals output by the N DFB lasers1302A-1302D immediately into a corresponding Si waveguide in a secondlayer of the Si PIC that is vertically displaced above or below thefirst layer, the SiN waveguides 1312A-1312D are optically coupled withinthe first layer of the Si PIC to a passive optical device 1314 includedin the first layer of the Si PIC of FIG. 13.

In the example of FIG. 13, the passive optical device 1314 includes aWDM component such as a WDM mux. The WDM mux may include a cascade ofMach-Zehnder (MZ) interferometers, an arrayed waveguide grating (AWG),an Echelle grating, or other suitable WDM mux. More generally, thepassive optical device 1314 may include any passive optical devicesuitable for formation in SiN.

The N optical signals output by the N DFB lasers 1302A-1302D aredirected by the SiN waveguides 1312A-1312D into the passive opticaldevice 1314. The passive optical device 1314 multiplexes the N opticalsignals into a multiplexed optical signal output to a common SiN outputwaveguide 1316 included in the first layer of the Si PIC of FIG. 13. Thecommon SiN output waveguide 1316 may be configured similar or identicalto other SiN waveguides described herein. The multiplexed optical signalis adiabatically coupled from the common SiN output waveguide 1316 intoa Si waveguide 1318 formed in the second layer of the Si PIC. Theadiabatic coupling is accomplished as described above, e.g., byproviding the Si waveguide 1318 with a tapered end that is aligned inthe two orthogonal dimensions with a coupler portion of the common SiNoutput waveguide 1316.

FIG. 14 is an overhead view of an example AWG 1400 that may be formed asa passive optical device such as a WDM component (e.g., a WDM mux or WDMdemux) using SiN in, e.g., the first layers 206, 710, 910, 1110 of theSi PICs 102, 702, 902, 1102, arranged in accordance with at least oneembodiment described herein. The first layer of the Si PIC may include aSiN waveguide 1402, the AWG 1400, and SiN waveguides 1404A-1404D. Aninterposer waveguide 1406 of an interposer forms an adiabatic couplerregion with the SiN waveguide 1402. A Si waveguide 1408A formed in asecond layer of the Si PIC forms an adiabatic coupler region with theSiN waveguide 1404A. Although not illustrated in FIG. 14, other Siwaveguides formed in the second layer of the Si PIC may form adiabaticcoupler regions with the other SiN waveguides 1404B-1404D.

In some embodiments, the AWG 1400 is a WDM demux, in which case amultiplexed optical signal is adiabatically coupled from the interposerwaveguide 1406 into the SiN waveguide 1402 and provided to the AWG 1400,which demultiplexes the multiplexed optical signal into multiple outputsignals (e.g., separate wavelength channels) separately output to theSiN waveguides 1404A-1404D. Each of the output signals may then beadiabatically coupled from the corresponding SiN waveguide 1404A-1404Dinto a corresponding Si waveguide, such as the Si waveguide 1408A in thecase of the SiN waveguide 1404A.

In some embodiments, the AWG 1400 is a WDM mux, in which case adifferent one of multiple input signals (e.g., separate wavelengthchannels) is adiabatically coupled from a corresponding Si waveguide,such as the Si waveguide 1408A or other Si waveguides of the Si PIC),into a corresponding SiN waveguide 1404A-1404D. The SiN waveguides1404A-1404D provide their respective input signal to the AWG 1400, whichmultiplexes the various input signals into a multiplexed optical signaloutput to the SiN waveguide 1402. The multiplexed optical signal maythen be adiabatically coupled from the SiN waveguide 1402 to theinterposer waveguide 1406.

In FIG. 14 (and in FIG. 13), each of the SiN waveguides 1402 and1404A-1404D may taper down from a relatively wide SiN waveguide to arelatively narrow SiN waveguide whose effective indexes for TE and TMare the same. Accordingly, the SiN-based AWG 1400 of FIG. 14 may bebased on a zero-birefringent SiN waveguide.

FIG. 15 is an overhead view of an example cascade of MZ interferometers1500 that may be formed as a passive optical device such as a WDMcomponent (e.g., a WDM mux) using SiN in, e.g., the first layers 206,710, 910, 1110 of the Si PICs 102, 702, 902, 1102, arranged inaccordance with at least one embodiment described herein. The cascade ofMZ interferometers 1500 may include or correspond to the passive opticaldevice 1314 of FIG. 13. Although the cascade MZ interferometer 1500 ofFIG. 15 is illustrated as a WDM mux that accepts N (N>=2) input opticalsignals and outputs one multiplexed optical signal, the cascade MZinterferometer 1500 may instead be implemented as a WDM demux thataccepts one multiplexed optical signal and outputs N individual opticalsignals.

The cascade of MZ interferometers 1500 may include a first stage 1502 ofMZ interferometers with a delay in one arm of each of the MZinterferometers of the first stage 1502 of ΔL, a second stage 1504 of MZinterferometers with a delay in one arm of each of the MZinterferometers of the second stage 1504 of 2·ΔL, and a third stage 1506with one MZ interferometer with a delay in one arm of the MZinterferometer of the third stage 1506 of 4·ΔL. An input to each MZinterferometer of each stage may include a 2×2 multimode interference(MMI) coupler and an output from each MZ interferometer of each stagemay include a 1×2 MMI coupler. The input of each MZ interferometer ofeach stage may alternatively include a 50/50 directional coupler.

The first stage 1502 of MZ interferometers may have inputs coupled toSiN waveguides 1508. Similar to the SiN waveguides 1312A of FIG. 13, theSiN waveguides 1508 of FIG. 15 may form adiabatic coupler regions withcorresponding interposer waveguides to adiabatically couple differentwavelength channels from a corresponding optical signal source, such asa corresponding one of the DFBs 1302A-1302D of FIG. 13, into the cascadeof MZ interferometers 1500.

The third stage 1506 of MZ interferometers may have an output coupled toan SiN waveguide 1510. Similar to the SiN waveguide 1316 of FIG. 13, theSiN waveguide 1510 of FIG. 15 may form an adiabatic coupler region witha Si waveguide to adiabatically couple a multiplexed output signal fromthe cascade of MZ interferometers 1500 into the Si waveguide.

FIG. 16 is a side view of another example two-stage adiabaticallycoupled photonic system 1600 (hereinafter “photonic system 1600”),arranged in accordance with at least one embodiment described herein.The photonic system 1600 includes a Si PIC 1602, the interposer 704, anda semiconductor chip 1604.

The Si PIC 1602 includes a Si substrate 1606, a SiO₂ box 1608, a firstlayer 1610 with one or more SiN waveguides 1612A, 1612B, and a secondlayer 1614 with one or more Si waveguides 1616A, 1616B. The Si substrate1606, the SiO₂ box 1608, the first layer 1610, the SiN waveguides 1612A,1612B, the second layer 1614, and the Si waveguides 1616A, 1616B maygenerally be similar or identical to, respectively, any of the other Sisubstrates, SiO₂ boxes, first layers, SiN waveguides, second layers, andSi waveguides disclosed herein except as otherwise indicated herein. Forinstance, the Si waveguide 1616B may be adiabatically coupled to the SiNwaveguide 1612A and the SiN waveguide 1612B may be adiabatically coupledto the interposer waveguide 730, in a similar manner as generallydescribed above. In some embodiments, the first layer 110 may include aWDM component and/or other features as described elsewhere herein.

The semiconductor chip 1604 may be wafer bonded to the Si PIC 1602 abovethe second layer 1614 of the Si PIC 1602. The semiconductor chip 1604may include an active optical device 1605, such as an InP-based gainelement or gain region needed to form a laser or an InP-based pindetector. The active optical device 1605 of the semiconductor chip 1604may be optically coupled to one or both of the Si waveguide 1616A or theSi waveguide 1616B. Alternatively, the Si waveguides 1616A and 1616B mayinclude opposite ends of the same Si waveguide. Accordingly, light maybe exchanged between the active optical device 1605 and one or both ofthe Si waveguides 1616A or 1616B. In an example implementation, the Siwaveguide 1616B includes a tapered end to adiabatically couple lightinto (or out of) the SiN waveguide 1612A, and an end of the Si waveguide1616B opposite its tapered end may include the Si waveguide 1616A whichmay be optically coupled to the active optical device 1605 of thesemiconductor chip 1604. A so-called hybrid laser structure can beformed by the InP gain element and Si by adding reflective distributedBragg reflectors (DBRs) in Si on either side of the InP gain region. TheSi DBRs in either side of the InP gain region form an optical cavitywith gain which hence produces a laser.

In some Si PICs described herein, the Si PIC may include metal layersand/or metallized structures for electrical contact to active opticalcomponents of the Si PIC. Such active optical components may befabricated at the so-called Back End of Line (BEOL) process. Further, tocouple light between the Si PIC and an interposer as described herein,an etched window through one or more upper layers down to a layer thatincludes a SiN waveguide may be formed to expose the SiN waveguide forcoupling to an interposer waveguide included in the interposer. In theseand other implementations, a top layer of the Si PIC may include metaldummies to maintain flatness after CMP. The metal dummies may have tomaintain a certain fill factor. The area of the etched window may bedetermined by the metal dummy fill factor and may be limited to a fewsquare millimeters (mm²).

FIG. 17 is a perspective view of an example Si PIC 1700 that defines anetched window 1702, arranged in accordance with at least one embodimentdescribed herein. The Si PIC 1700 includes a first layer 1710 withvarious SiN waveguides 1712, tapered ends of which are visible in theetched window 1702. The Si PIC 1700, the etched window 1702, the firstlayer 1710, and the SiN waveguides 1712 may generally be similar oridentical to other Si PICs, etched windows, first layers, and SiNwaveguides disclosed herein except as otherwise indicated herein. The SiPIC 1700 may additionally include one or more other components orelements similar to those described with respect to one or more of theother Si PICs disclosed herein.

The Si PIC 1700 additionally includes one or more dielectric layers 1722above the first layer 1710, which dielectric layers may be similar oridentical to other dielectric layers disclosed herein. The etched window1702 may be formed by etching through the dielectric layers 1722 to thefirst layer 1710. Accordingly, the etched window 1702 may be bounded onthree sides (two of which are visible in FIG. 17) by the dielectriclayers 1722. At least a topmost one of the dielectric layers 1722includes metal dummies 1704 at least in a region that bounds the etchedwindow 1702 on the three sides. Alternatively, the metal dummies 1704may extend from the topmost one of the dielectric layers 1722 downwardthrough up to all of the dielectric layers 1722 or some portion thereof.

In an example embodiment, each of the tapered ends of the SiN waveguides1712 may be about 2.2 millimeters (mm) long such that the etched window1702 is also at least that long, the dielectric layers 1722 may be about5-6 μm thick such that the etched window 1702 is etched through thedielectric layers 1722 at least that deep, the SiN waveguides 1712 mayhave a pitch of about 50 μm, and the etched window 1702 may have a widthof 400 μm. Other particular values are possible depending on the desiredimplementation.

FIG. 18 includes a bottom view 1800A and a side view 1800B of animplementation of a portion of the interposer 704 that may be coupled tothe Si PIC 1700 of FIG. 17 within the etched window 1702, arranged inaccordance with at least one embodiment described herein. In theembodiment of FIG. 18, the interposer 704 includes the interposersubstrate 726 and the waveguide strip 728 coupled thereto. The waveguidestrip 728 includes multiple interposer waveguides 730, each of whichincludes interposer core 730A and interposer cladding 730B. In theexample of FIG. 18, the interposer 704 may include a polymer interposersuch that the interposer substrate 726, the interposer cores 730A, andthe interposer cladding 730B respectively include a polymer substrate,polymer cores, and polymer cladding.

A thickness t_(is) of the interposer substrate 726 may be greater thanor equal to about 100 μm. A thickness t_(clad) of the interposercladding 730B may be about 14 μm. A pitch p of the interposer cores730A, e.g., a nominal core center-to-core center spacing of theinterposer cores 730A, may be about 50 μm, or more generally X μm. Awidth w_(core) of each of the interposer cores 730A may be about 8 μm. Athickness t_(core) of each of the interposer cores 730A may be less thanor equal to a depth of a corresponding etched window of a correspondingSi PIC to which the interposer 704 is to be coupled. A width w_(ws) ofthe waveguide strip 728 may be about N times X, where N is a number ofthe interposer cores 730A and X is the pitch p or nominal corecenter-to-core center spacing. A minimum width of the correspondingetched window may also be N times X. Other particular values arepossible depending on the desired implementation.

In the views 1800A and 1800B of FIG. 18, the interposer cores 730Ainclude coupler portions of the polymer waveguides 730. The couplerportions visible in FIG. 18 may be aligned as described above withtapered ends of corresponding SiN waveguides accessible through thecorresponding etched window. The coupler portions are exposed on threeof four sides along their length with the interposer cladding 730B beingdisposed adjacent to the remaining one of the four sides along thelength of the coupler portions. Alternatively, the coupler portions maybe exposed only on a bottom side, or along the bottom side and onlypartially on one or both vertical sides. In these and other embodiments,for a portion (not shown) of the interposer 704 that is not to bedisposed inside a corresponding etched window, however, the interposercores 730A may generally be surrounded on all four sides along theirlength by the interposer cladding 730B.

FIGS. 19A and 19B are side views that depict alignment and attachment ofthe interposer 704 of FIG. 18 and the Si PIC 1700 of FIG. 17, arrangedin accordance with at least one embodiment described herein. Asillustrated in FIG. 19A, the waveguide strip 728 of the interposer 704is aligned to the etched window 1702 with the interposer cores 730Agenerally aligned in the x and z directions with the SiN waveguides 1712to form adiabatic coupler regions as described elsewhere herein. Theetched window 1702 may be at least partially filled with an epoxyunderfill 1902. The interposer 704 may then be moved towards the Si PIC1700 (or vice versa) as indicated by the arrow 1904 in FIG. 19A untilthe interposer cores 730A are in direct or at least close contact withthe SiN waveguides 1712, as illustrated in FIG. 19B. As used herein,direct contact between two components or elements means the twocomponents are actually touching each other. Close contact as usedherein means the two components are sufficiently close for light to beoptically coupled from one component to the other. Such components inclose contact may optionally include between the two components an epoxyor other adhesive. Any descriptions herein referring to direct contactcan also include close contact which may include a thin layer of, e.g.,adhesive. As illustrated in FIG. 19B, there may be sufficient underfillepoxy 1902 to overflow the etched window 1902 so as to epoxy the top ofthe dielectric layers 1722 to the interposer cladding 730B of theinterposer 704.

FIGS. 19A and 19B additionally illustrate Si waveguides 1906 included inthe Si PIC 1700 that may generally be aligned in the x and z directionswith the SiN waveguides 1712 to form adiabatic coupler regions asdescribed elsewhere herein.

FIG. 20 is a side view that depicts alignment of another interposer 2002and Si PIC 2004, arranged in accordance with at least one embodimentdescribed herein. The example of FIG. 20 implements a multiple windowgeometry to satisfy maximum window size and metal dummy fill factorconstraints and may otherwise be similarly configured to otherembodiments discussed above, including implementation of two-stageadiabatic coupling as discussed herein. In this and other embodiments,the interposer 2002 may include multiple waveguide strips 2006 and theSi PIC 2004 may include multiple etched windows 2008. Each of thewaveguide strips 2006 and etched windows 2008 may generally be similaror identical to any of the other waveguide strips and etched windowsdisclosed herein. In general, a lower surface of the interposer 2002, atleast in a region where the interposer 2002 couples to the Si PIC 2004,may be complementary to an upper surface of the Si PIC 2004, at least ina region where the Si PIC 2004 couples to the polymer interposer 2002.

FIG. 21 is a side view that depicts alignment of another interposer 2102and Si PIC 2104, arranged in accordance with at least one embodimentdescribed herein. The interposer 2102 includes one or more waveguidestrips 2106 while the Si PIC 2104 includes one or more etched windows2108. In addition, the example of FIG. 21 implements one or moreinterposer alignment ridges 2110 and corresponding Si PIC anchor windows2112 and/or one or more dummy interposer islands 2114 and is otherwisesimilarly configured to other embodiments discussed above, includingimplementation of two-stage adiabatic coupling as discussed herein.

The interposer alignment ridges 2110 in some embodiments may be formedfrom the same material as the interposer cores, interposer cladding, orinterposer substrate of the interposer 2102. Alternately oradditionally, each of the interposer alignment ridges 2110 may be about100 to 200 μm wide and the same or a different thickness as theinterposer cores.

The anchor windows 2112 may be etched through one or more dielectriclayers of the Si PIC 2104 that are above a corresponding first layer ofthe Si PIC 2104 that includes SiN waveguides 2116 that are to beoptically coupled to interposer waveguides included in the waveguidestrip 2106. The shapes and locations of the anchor windows 2112 may becomplementary to the shapes and locations of the interposer alignmentridges 2110. When attaching the polymer interposer 2102 to the Si PIC2104, the interposer alignment ridges 2110 may be aligned to the anchorwindows 2104, which may in turn align exposed coupler portions of theinterposer waveguides of the waveguide strip 2106 to the SiN waveguides2116. The interposer 2102 may then be moved towards the Si PIC 2104 (orvice versa) as indicated by the arrows 2118 in FIG. 21 until theinterposer cores are in direct or at least close contact with the SiNwaveguides 2116 to form corresponding adiabatic coupler regions.

The dummy interposer islands 2114 in some embodiments may be formed fromthe same material as the interposer cores, interposer cladding, orinterposer substrate of the interposer 2102. Alternately oradditionally, each of the dummy interposer islands 2114 may be the sameor a different width as the interposer alignment ridges 2110 and thesame or a different thickness as the interposer cores. A width of theetched window 2108 may be sufficient to accommodate therein the dummyinterposer islands 2114 and the waveguide strip 2106 (or moreparticularly the coupler portions of the interposer waveguides includedtherein). The dummy interposer islands 2114 may be separated from thenearest interposer waveguides by a sufficient distance to not perturbthe optical mode in the nearest interposer waveguides. For example, eachof the dummy interposer islands 2114 may be separated from acorresponding nearest interposer waveguide of the waveguide strip 2106by at least 30 μm. In general, the dummy interposer islands 2114 mayprovide a relatively large and flat surface to facilitate mechanicalattachment process between the interposer 2102 and the Si PIC 2104.

FIG. 22 includes a side view 2200A and a bottom view 2200B of anotherarrangement of an interposer 2202 with interposer alignment ridges 2204and dummy interposer islands 2206, arranged in accordance with at leastone embodiment described herein. Similar to other interposers disclosedherein, the interposer 2202 may include an interposer substrate 2208, aninterposer cladding 2210, and interposer cores 2212. In someembodiments, the interposer 2202 includes a polymer interposer in whicheach of the interposer substrate 2208, the interposer cladding 2210, andthe interposer cores 2212 include polymer. As illustrated in the bottomview 2200B, the interposer cladding 2210 may be removed from a bottomand/or sides of the interposer waveguides 2212 at least in a region 2214of the interposer 2202 to be received in an etched window of an Si PIC.In a region 2216 of the interposer 2202 that is not received in theetched window, the interposer cladding 2210 may surround all sides ofthe interposer waveguides 2212 along their length.

FIG. 23A is a side view of another example two-stage adiabaticallycoupled photonic system 2300 (hereinafter “photonic system 2300”) thatincludes a Si PIC 2302, an interposer 2304, and an optical fiber endconnector 2306 (hereinafter “connector 2306”), arranged in accordancewith at least one embodiment described herein. The Si PIC 2302 and theinterposer 2304 may be similar or identical to, respectively, any of theother Si PICs and interposers disclosed herein except as otherwiseindicated herein.

For example, the Si PIC 2302 may include one or more SiN waveguides 2308formed in a first layer of the Si PIC and one or more Si waveguides 2310formed in a second layer of the Si PIC that is below (or above in otherembodiments) the first layer. Each of the Si waveguides 2310 may includea tapered end aligned in two orthogonal directions with a couplerportion of a corresponding one of the SiN waveguide 2308 to form anadiabatic coupler region. Analogously, each of the SiN waveguides 2308may include a tapered end aligned in the two orthogonal directions witha coupler portion of a corresponding one of one or more interposerwaveguides 2312 included in the interposer 2304 to form anotheradiabatic coupler region.

The interposer 2304 may include a high index glass waveguide block orhigh index glass waveguide interposer. Accordingly, in this example, theinterposer waveguides 2312 may include high index glass waveguides thatmay be written into the high index glass waveguide block, e.g., by ionexchange method, ultraviolet (UV) radiation laser writing, or othersuitable index altering radiation or process.

Each of the interposer waveguides 2312 may generally be aligned relativeto a corresponding one of the SiN waveguides 2308 actively or passivelyto form adiabatic coupler regions. The alignment of each of the SiNwaveguides 2308 to the corresponding Si waveguide 2310 may be achievedin the fabrication process to form adiabatic coupler regions.

An epoxy underfill 2314 may be provided between the interposer 2304 andthe Si PIC 2302 to form a mechanical attachment therebetween.

The connector 2306 may include a multi-fiber push on (MPO) connector orother suitable optical fiber end connector.

The interposer 2304 may be coupled to the connector 2306, which in turnmay be coupled to one or more optical fibers (not shown). Light may becoupled from the optical fibers into the interposer waveguides 2312 ofthe interposer 2304, and/or from the interposer waveguides 2312 of theinterposer 2304 into the optical fibers.

FIG. 23B is a perspective view of the interposer 2304 of FIG. 23A,arranged in accordance with at least one embodiment described herein. Inthese and other implementations, the interposer 2304 may include one ormore alignment guides or threaded openings 2316 to receive protrusionsor threaded fasteners of the connector 2306 to couple the connector 2306to the interposer 2304 and/or to optically align the interposerwaveguides 2312 of the interposer 2304 with the optical fibers.

In some implementations, the interposer waveguides 2312 may be dividedinto two or more subsets or groups. In the example of FIG. 23B, theinterposer waveguides 2312 are divided into a first subset 2318A ofinterposer waveguides 2312 and a second subset 2318B of interposerwaveguides 2312. The interposer waveguides 2312 may be divided accordingto their intended function. For instance, the first subset 2318A ofinterposer waveguides 2312 may be used to carry incoming light from theoptical fibers through the connector 2306 to the Si PIC 2302, and thusmay be referred to as receive (RX) interposer waveguides 2312.Analogously, the second set 2318B of interposer waveguides 2312 may beused to carry outgoing light from the Si PIC 2302 to the optical fibersthrough the connector 2306, and thus may be referred to as transmit (TX)interposer waveguides 2312. Si waveguides in the second layer of the SiPIC 2302 and/or SiN waveguides in the first layer of the Si PIC 2302 mayalso be described as being RX or TX waveguides depending on the functionthey serve.

As illustrated in FIG. 23B, at an input/output surface 2320 of theinterposer 2304, ends of the RX interposer waveguides 2312 in the firstset 2318A may generally be arranged parallel to each other and coplanar,while ends of the TX interposer waveguides 2312 in the second set 2318Bmay also generally be arranged parallel to each other and coplanar.Alternately or additionally, at the input/output surface 2320, the endsof the RX interposer waveguides 2312 of the first set 2318A may bedisplaced from and parallel to the ends of the TX interposer waveguides2312 of the second set 2318B in a double-decker arrangement, asillustrated in FIG. 23B.

The input/output surface 2320 of the interposer 2304 may be coupled tothe connector 2306 of FIG. 23A. The double-decker arrangement of the RXinterposer waveguides 2312 of the first set 2318A and the TX interposerwaveguides 2312 of the second set 2318B at the input/output surface 2320may match an arrangement of RX optical fibers and TX optical fibers towhich the connector 2306 of FIG. 23A may be coupled. Other arrangementsof the RX and TX interposer waveguides 2312 may be implemented to matchother arrangements of the RX and TX optical fibers through the connector2306.

FIG. 24 is a perspective view of another example photonic system 2400(hereinafter “photonic system 2400”) that includes a Si PIC 2402, aninterposer 2404, and an optical fiber end connector 2406, arranged inaccordance with at least one embodiment described herein. The photonicsystem 2400 additionally includes a Si PIC 2402 and an optical fiber endconnector 2406. The Si PIC 2402, the interposer 2404, and the connector2406 may be similar or identical to, respectively, any of the other SiPICs, interposers, and connectors disclosed herein except as otherwiseindicated herein.

For example, the Si PIC 2402 may include one or more SiN waveguides 2408formed in a first layer of the Si PIC and one or more Si waveguides (notshown) formed in a second layer of the Si PIC that is below (or above inother embodiments) the first layer. Each of the Si waveguides mayinclude a tapered end aligned in two orthogonal directions with acoupler portion of a corresponding one of the SiN waveguide 2408 to forman adiabatic coupler region. Analogously, each of the SiN waveguides2408 may include a tapered end aligned in the two orthogonal directionswith a coupler portion of a corresponding one of one or more interposerwaveguides included in the interposer 2404 to form another adiabaticcoupler region.

The interposer 2404 may include a polymer interposer with a flexiblepolymer substrate and one or more polymer waveguides formed thereon. Thepolymer waveguides of the interposer 2404 may be divided into a firstsubset of RX polymer waveguides and a second subset of TX polymerwaveguides, with the ends of the polymer waveguides arranged in a doubledecker arrangement where they connect to the connector 2406, similar tothe double decker arrangement described with respect to FIG. 23B.

In general, light may be coupled out of or coupled into the Si PIC 2402at a planar interface of the Si PIC 2402, e.g., a SiN/SiO₂ layer of theSi PIC 2402 that includes the SiN waveguides 2408 of the Si PIC 2402.Positions of the tapered ends of the SiN waveguides 2408 in the Si PIC2402, and thus of tapered ends of the Si waveguides of the Si PIC 2402may be offset for RX Si waveguides as compared to TX Si waveguides inthe light propagation direction to better isolate incoming and outgoinglight from each other.

For example, FIGS. 25A and 25B illustrate two different offsetconfigurations for RX vs. TX SiN waveguides, arranged in accordance withat least one embodiment described herein. In each of FIGS. 25A and 25B,tapered ends of RX SiN waveguides RX1 and RX2 terminate at a common zlocation (hereinafter “first z location”) and tapered ends of TX SiNwaveguides TX1 and TX2 terminate at a different common z location(hereinafter second z location) than for RX1 and RX2. In FIG. 25A, thetapered ends of the RX Si waveguides alternate with the tapered ends ofthe TX Si waveguides. In comparison, in FIG. 25B, the tapered ends ofthe RX SiN waveguides as a group are located next to the tapered ends ofthe TX SiN waveguides as a group.

Due to the z offset in FIGS. 25A and 25B between the RX and TX SiNwaveguides, RX and TX portions of an interposer to couple light into orout of a Si PIC that includes the RX and TX SiN waveguides of FIGS.25A-25B may be separated from each other. For instance, RX interposerwaveguides of the interposer may be coupled to the Si PIC in a regiongenerally denoted at 2502A in FIG. 25A and 2502B in FIG. 25B, while TXinterposer waveguides of the interposer may be coupled to the Si PIC ina region generally denoted at 2504A in FIG. 25A and 2504B in FIG. 25B.Although FIGS. 25A and 25B are discussed in the context of SiNwaveguide/interposer waveguide adiabatic coupler regions, the sameprinciples may be applied for Si waveguide/interposer waveguideadiabatic coupler regions.

Some interposers discussed herein have been described as includingpolymer or high index glass. Other materials for the interposer arepossible. For example FIG. 26 includes a side view 2600A and a bottomview 2600B of a silicon oxynitride (SiON) interposer 2602, arranged inaccordance with at least one embodiment described herein.

The SiON interposer 2602 includes a SiON waveguide strip 2604 withmultiple SiON waveguides 2606, each including a SiON core 2608 and aSiON cladding 2610. The SiON cores 2608 may be exposed (e.g., notsurrounded by SiON cladding 2610) on at least one surface within acoupling region of the SiON interposer 2602 to be received in an etchedwindow of a corresponding Si PIC so as to be aligned and brought intodirect or at least close contact with corresponding SiN waveguides ofthe Si PIC.

In the illustrated embodiment, the SiON interposer 2602 includes SiON onSiO₂ substrate 2612 or other substrate. SiON has a refractive index thatcan vary between that of SiO₂ around 1.46 and that of SiN around 1.99 bychanging the growth conditions of the fraction of 0 and N in the SiONportions of the SiON interposer 2602. A refractive index of around 1.51can be achieved to form the SiON cladding 2610 and a slightly higherindex of 1.516, for example, can be achieved to form the SiON cores 2608of the SiON waveguides 2606.

A width w_(s) of the SiO₂ substrate 2612 may be in a range from 2 mm to7 mm. A pitch H p of the SiON cores 2608 may be in a range from 50 μm to250 μm. A width ws of the waveguide strip 2604 may be in a range from400 μm to 1.5 mm depending on number of SiON cores 2608 and the pitch p.A thickness t of the SiON cladding 2610 may be greater than or equal to15 μm. A thickness t_(core) and a width w_(core) of the SiON cores 2608may each be in a range from 6 μm to 8 μm. Other particular values arepossible depending on the desired implementation.

In the example of FIG. 26, the SiON cladding 2610 can be flush with atop surface (in a growth direction) of the SiON cores 2608 (which is thebottom surface in the orientation of the view 2600A of FIG. 26. The SiONwaveguides 2606 may be aligned in two orthogonal directions withcorresponding SiN waveguides of a Si PIC to form adiabatic couplerregions. The SiON of the SiON interposer 2602 can be etched to form aplug to fit a corresponding etched window in a Si PIC, such as isillustrated in FIG. 27.

FIG. 27 is a side view that depicts alignment of the SiON interposer2602 of FIG. 26 and the Si PIC 1700 of FIG. 17, arranged in accordancewith at least one embodiment described herein. As illustrated in FIG.27, the SiON waveguide strip 2604 of the SiON interposer 2602 is alignedto the etched window 1702 of the Si PIC 1700 with the SiON cores 1608generally aligned in the x and z directions with the SiN waveguides 1712of the Si PIC 1700 in the manner described above to form adiabaticcoupler regions. The etched window 1702 may be at least partially filledwith the epoxy underfill 1902. The SiON interposer 2602 may then bemoved towards the Si PIC 1700 (or vice versa) as indicated by the arrow2702 until the SiON cores 2608 are in direct or at least close contactwith the SiN waveguides 1712 of the Si PIC 1700.

FIG. 28 illustrates two example optoelectronic systems 2800A and 2800B(hereinafter “systems 2800”) that each include at least one polymer onglass interposer 2802A, 2802B, 2802C (collectively “polymer on glassinterposers 2802”), arranged in accordance with at one embodimentdescribed herein. The polymer on glass interposers 2802 may generally besimilar or identical to any of the other interposers disclosed hereinexcept as otherwise indicated herein. Each of the systems 2800 includesa multi-channel optoelectronic module (hereinafter “module”) 2804A or2804B, such as a 4-channel parallel single mode 4 (PSM4) transceiver.Each of the modules 2804A and 2804B includes a Si PIC with one or moreSi waveguides and one or more SiN waveguides that together form one ormore adiabatic coupler regions.

In the photonic system 2800A, the module 2804A is configured to receivemultiple optical signals 2806A from an optical network through an inputconnector 2808A. The optical signals 2806A may be adiabatically coupledinto the Si PIC of the module 2804A through the polymer on glassinterposer 2802A and one or more SiN waveguides and Si waveguides of theSi PIC of the module 2804A, in the manner generally described above.

In the photonic system 2800B, the module 2804B is configured to transmitmultiple optical signals 2806B to the optical network through an outputconnector 2808B. One or more of the optical signals 2806B may beadiabatically coupled from an optical transmitter 2810 of the module2804B and into the Si PIC of the module 2804A through the polymer onglass interposer 2802C (labeled “Polymer on glass plug”) and one or moreSiN waveguides and Si waveguides of the Si PIC of the module 2804B, inthe manner generally described above. The optical signals may also beadiabatically coupled out of the Si PIC and into the output connector2806B through one or more Si waveguides and SiN waveguides of the Si PICof the module 2804B and the polymer on glass interposer 2802B, in themanner generally described above.

FIG. 29A illustrates an example polymer on glass interposer 2900A and SiPIC 2902, arranged in accordance with at least one embodiment describedherein. The polymer on glass interposer 2900A may be implemented in,e.g., either or both of the systems 2800 of FIG. 28 as one or more ofthe polymer on glass interposers 2802A-2802C.

In the illustrated embodiment, the Si PIC 2902 defines an etched window2904. The Si PIC 2902 additionally includes a Si substrate 2906, a SiO₂box 2908, a first layer 2910 with various SiN waveguides 2912, a secondlayer 2914 with various Si waveguides 2916, and one or more dielectriclayers 2918 above the first layer 2910 that includes the SiN waveguides2912. The Si PIC 2902, the etched window 2904, the Si substrate 2906,the SiO₂ box 2908, the first layer 2910, the SiN waveguides 2912, thesecond layer 2914, the various Si waveguides 2916, and the dielectriclayers 2918 may generally be similar or identical to, respectively, anyof the other Si PICs, etched windows, Si substrates, SiO₂ boxes, firstlayers, SiN waveguides, second layers, Si waveguides, and dielectriclayers disclosed herein excepted as otherwise indicated herein. Forexample, the SiN waveguides 2912 and the Si waveguides 2916 may bearranged relative to each other to adiabatically couple light from theSi waveguides 2916 to the SiN waveguides 2912, or vice versa, asdescribed elsewhere herein. The Si PIC 2902 may additionally include oneor more other components, layers, features, or aspects as describedelsewhere herein.

The etched window 2904 may be formed by etching through the dielectriclayers 2918 to the second layer 2914. In some embodiments, the etchedwindow 2904 is bounded on three sides (two of which are visible in FIG.29A) by the dielectric layers 2918. At least a topmost one of thedielectric layers 2918 includes metal dummies 2920 at least in a regionthat bounds the etched window 2904 on the three sides.

The polymer on glass interposer 2900A includes a glass substrate 2922and a polymer waveguide strip coupled thereto. The glass substrate 2922may include UV transparent glass and is a specific example of aninterposer substrate. The polymer waveguide strip is a specific exampleof a waveguide strip and includes multiple polymer waveguides 2924, eachof which includes a polymer core 2926 and polymer cladding 2928. Thepolymer cladding layer 2928 is coupled to the glass substrate 2922. Thepolymer cores 2926 are coupled to the polymer cladding 2928. The polymerwaveguides 2924 include coupler portions as described above that areconfigured to be aligned in two orthogonal directions (e.g., x and zdirections) with tapered ends of the SiN waveguides 2912 such that thecoupler portions of the polymer waveguides 2924 overlap in the twoorthogonal directions and are parallel to the tapered ends of the SiNwaveguides 2912. In this arrangement, light may be adiabatically coupledfrom the SiN waveguides 2912 to the polymer waveguides 2924, or viceversa.

As illustrated, the polymer cores 2926 are parallel to each other. Thepolymer cores 2926 may have a pitch of 250 micrometers. Alternatively,the pitch of the polymer cores 2926 may be in a range from 290-500micrometers, or some other value. A length of the polymer cores 2926and/or of the polymer on glass interposer 2900A in the z direction maybe in a range from 1 millimeter to 4 millimeters, at least for a portionof the length of the polymer cores 2926 that is received within theetched window 2904. A height or thickness in the y direction of each ofthe polymer cores 2926 may be less than or equal to a depth in the ydirection of the etched window 2904. In other embodiments, the height orthickness in the y direction of each of the polymer cores 2926 may begreater than the depth in the y direction of the etched window 2904. Inan example embodiment, the height of the polymer cores 2926 is in arange from 4 μm to 7 μm. A width in the x direction of the polymer onglass interposer 2900A may be in a range from 1 mm to 2 mm.

In some embodiments, the etched window 2904 may be at least partiallyfilled with an epoxy underfill 2930. To assemble the polymer on glassinterposer 2900A and the Si PIC 2902 together, the polymer on glassinterposer 2900A may be moved toward the Si PIC 2902 as indicated byarrow 2932 until the polymer cores 2926 are in direct or at least closecontact with the SiN waveguides 2912. In some embodiments, there may besufficient epoxy underfill 2930 to overflow the etched window 2904 so asto epoxy the top of the dielectric layers 2918 to the polymer cladding2928 of the polymer on glass interposer 2900A.

FIG. 29B illustrates another example polymer on glass interposer 2900B,arranged in accordance with at least one embodiment described herein.The polymer on glass interposer 2900A may be implemented in, e.g.,either or both of the systems 2800 of FIG. 28 as one or more of thepolymer on glass interposers 2802A-2802C.

The polymer on glass interposer 2900B includes the glass substrate 2922and the polymer waveguides 2924, including the polymer cores 2926 andthe polymer cladding 2928. The polymer on glass interposer 2900Badditionally includes one or more first polymer alignment ridges 2934Adisposed to a first side of the polymer waveguides 2924 and one or moresecond polymer alignment ridges 2934B disposed to a second side of thepolymer waveguides 2924 that is opposite the first side. The polymeralignment ridges 2934A and 2934B (collectively “polymer alignment ridges2934”) may be received in one or more corresponding etched channels,windows, recesses, or other features of a corresponding Si PIC to alignthe polymer on glass substrate 2900B (and more particularly, the polymerwaveguides 2924) to the Si PIC (and more particularly, SiN waveguides ofthe Si PIC).

The polymer on glass interposers 2900A and 2900B of FIGS. 29A and 29Band the Si PIC 2902 of FIG. 29A may include one or more othercomponents, layers, features, or aspects as described elsewhere herein.

For example, the polymer on glass substrate 2900B may further includeone or more dummy polymer islands, such as a first dummy polymer islandbetween the polymer cores 2926 and the first polymer alignment ridges2934A, and a second dummy polymer island between the polymer cores 2926and the second polymer alignment ridges 2934B. In these and otherembodiments, a width of the etched window of the Si PIC 2902 may besufficient to accommodate therein the first dummy polymer island, thecoupler portion of each of the polymer waveguides 2924, and the seconddummy polymer island.

Referring again to FIGS. 3A and 3B, and as already described, light maybe coupled from the Si waveguide 212 to the SiN waveguide 208 and thenfrom the SiN waveguide 208 to the interposer waveguide 224. The Sisubstrate (not shown) on which the SiO₂ box 204 is formed is somedistance d (e.g., in the y direction) away from the SiN waveguide 208.Here, the distance d is about equal to the thickness of the SiO₂ box 204plus the thickness of the second layer 210. In an example embodiment,the thickness of the SiO₂ box 204 is 0.72 micrometers and the thicknessof the second layer 210 is about 0.3 micrometers such that the distanced may be about 1.02 micrometers. For these values, some lightpropagating in the SiN waveguide 208 may couple into the Si substrateand be lost. This loss may be referred to as substrate leakage. Thesubstrate leakage may be significant since an optical mode in the SiNwaveguide 208 may be much less confined than in the Si waveguide 212.

Some embodiments described herein reduce the substrate leakage byincreasing the distance d between the SiN waveguide 208 and the Sisubstrate. For example, the thickness of the SiO₂ box 204 may beincreased to a thickness greater than 0.72 micrometers, such as 2micrometers, or a thickness in a range of 2 micrometers plus or minus10%. However, increasing the thickness of the SiO₂ box 204 to such anextent may be incompatible with some fabs/manufacturers.

Alternatively, one or more other modifications may be made. Forinstance, the thickness of the SiN waveguide 208 in the y direction maybe increased to better confine a vertical E-field of propagating lightand therefore reduce substrate leakage. Alternatively or additionally,an SiO₂ layer may be provided between the first layer 206 and the secondlayer 210 and/or the thickness of such a layer may be increased toincrease the distance d between the SiN waveguide 208 and the Sisubstrate. As the distance d increases, Si—SiN TE coupling may decreaseto decrease substrate leakage. The foregoing will be described withrespect to FIG. 30. Alternatively or additionally, a two-layer SiNstructure may be implemented as described with respect to FIGS. 31A and31B.

FIG. 30 illustrates a cross-sectional view of an example Si PIC 3000,arranged in accordance with at least one embodiment described herein.The Si PIC 3000 may generally be similar or identical to any of theother Si PICs disclosed herein except as otherwise indicated herein. Thecross-sectional view of FIG. 30 is taken from a similar perspective asthe cross-sectional view 300C of FIG. 3B and illustrates an examplelayer stackup of the Si PIC 3000. The Si PIC 3000, as compared to theexample of FIGS. 3A and 3B, increases a thickness of a SiN waveguide andincreases a distance between the SiN waveguide and a corresponding Sisubstrate to reduce substrate leakage.

As illustrated, the Si PIC 3000 includes a Si substrate 3002, a SiO₂ box3004, a first layer 3006 that includes a SiN waveguide 3008, a SiN slab3010, and a second layer 3012 that includes a Si waveguide 3014. The SiPIC 3000 may additionally include a first SiO₂ layer 3016 between thesecond layer 3012 and the SiN slab 3010 and a second SiO₂ layer 3018between the SiN slab 3010 and the first layer 3006. The Si waveguide3014 and the SiN waveguide 3008 may be arranged to form an adiabaticcoupler region as described elsewhere herein.

In some embodiments, a total thickness of all layers of the Si PIC 3000between a top of the Si substrate 3002 and a bottom of the first layer3006 that includes the SiN waveguide 3008 may be at least 1.2 μm. Forexample, the SiO₂ box 3004 may have a thickness of 0.72 μm, or athickness in a range of 0.72 μm plus or minus 10%, or some otherthickness. The SiN waveguide 3008, and thus the first layer 3006, mayhave a thickness of 0.7 μm, or a thickness in a range of 0.7 μm plus orminus 10%, or some other thickness. The second SiO₂ layer 3018immediately beneath the SiN waveguide 3008 may have a thickness of atleast 0.1 μm, or a thickness in a range of 0.1 μm to 0.2 μm or more, orsome other thickness. The Si waveguide 3014, and thus the second layer3012, may have a thickness of 0.3 μm, or a thickness in a range of 0.3μm plus or minus 10%, or some other thickness. The first SiO₂ layer 3016may be omitted altogether, or may have a thickness in a range from 10nm-290 nm. The SiN slab 3010 may be omitted altogether or may have athickness in a range of 0.04 μm to 0.07 μm, or some other thickness.Accordingly, in some embodiments, all the layers between the Sisubstrate 3002 and the first layer 3006 may have a total thickness of atleast 1.2 μm (e.g., 0.72+0.2+0.3=1.22 μm) in the example of FIG. 30, ascompared to about 1 μm in the example of FIGS. 3A and 3B.

As compared to the example of FIGS. 3A and 3B, the optical mode may bemore confined in the relatively larger SiN waveguide 3008. Additionally,the increased distance between the Si substrate 3002 and the SiNwaveguide 3008 as compared to FIGS. 3A and 3B may further opticallyisolate the Si substrate 3002 from the SiN waveguide 3008 to reducesubstrate leakage.

FIG. 30 additionally illustrates first-third simulations 3020A-3020C forthe Si PIC 3000 of FIG. 30 in which SiN propagation loss through the SiNwaveguide 3008 has been ignored. The first simulation 3020A includes agraph of propagation loss or substrate leakage along the vertical axisin units of decibels (dB) per centimeter (cm) as a function of SiO₂ gapthickness along the horizontal axis in units of nanometers. The SiO₂ gapthickness in the first simulation 3020A refers to the thickness of thesecond SiO₂ layer 3018 in the Si PIC 3000. As illustrated in the firstsimulation 3020A, propagation loss of TM and TE optical modes (labeled“TM” and “TE” throughout FIG. 30) decreases with increasing SiO₂ gapthickness. For instance, from a SiO₂ gap thickness of 0.1 μm to 0.2 μm,the propagation loss for the TM optical mode decreases from about 1.16dB/cm to about 0.55 dB/cm and the propagation loss for the TE opticalmode decreases from H about 0.91 dB/cm to about 0.45 dB/cm.

The second simulation 3020B includes a graph of SiN-to-Si couplingefficiency along the vertical axis as a function of Si taper lengthalong the horizontal axis in units of μm. The Si taper length refers toa length of a tapered end of the Si waveguide 3014. As illustrated inthe second simulation 3020B, the SiN-to-Si coupling efficiency generallyincreases with increasing Si taper length and is about 97% or higher forboth TE and TM optical modes at a Si taper length of about 250 μm.

The third simulation 3020C includes a graph of polymer-to-SiN couplingefficiency along the vertical axis as a function of SiN linear taperlength along the horizontal axis in units of μm. The SiN linear taperlength refers to a length of the tapered end of the SiN waveguide 3008.As illustrated in the third simulation 3020C, the polymer-to-SiNcoupling efficiency generally increases with increasing SiN linear taperlength and is about 95% or higher for both TE and TM optical modes at aSiN linear taper length of about 2 millimeters (or 2000 μm).

The Si PIC 3000 may include one or more other components, layers,features, or aspects as described elsewhere herein.

FIG. 31A illustrates another example Si PIC 3100, arranged in accordancewith at least one embodiment described herein. The Si PIC 3100 maygenerally be similar or identical to any of the other Si PICs disclosedherein except as otherwise indicated herein. FIG. 31A includes across-sectional view 3101A and an overhead view 3101B of the Si PIC3100. The cross-sectional view of FIG. 31A is taken from a similarperspective as the cross-sectional view 300C of FIG. 3B and illustratesan example layer stackup of the Si PIC 3100. The Si PIC 3100 implementsa two-layer SiN structure to reduce substrate leakage.

As illustrated, the Si PIC 3100 includes a Si substrate 3102, a SiO₂ box3104, a first layer 3106 that includes a SiN waveguide 3108, a SiN slab3110, a second layer 3112 that includes a Si waveguide 3114, and a thirdlayer 3116 that includes a SiN transition waveguide 3118. The Si PIC3100 may additionally include one or more SiO₂ layers 3120 between thesecond layer 3112 and the SiN slab 3110, between the SiN slab 3110 andthe third layer 3116, and/or between the third layer 3116 and the firstlayer 3106.

In some embodiments, a total thickness of all layers of the Si PIC 3100between a top of the Si substrate 3102 and a bottom of the first layer3106 that includes the SiN waveguide 3108 may be at least 1.2 μm, suchas 1.6 μm or 1.6 μm plus or minus 10%. In more detail, the SiO₂ box 3104may have a thickness of 0.72 μm, or a thickness in a range of 0.72 μmplus or minus 10%, or some other thickness. The Si waveguide 3114, andthus the second layer 3112, may have a thickness of 0.3 μm, or athickness in a range of 0.3 μm plus or minus 10%, or some otherthickness. The SiO₂ layer 3120 immediately above the second layer 3112may be omitted altogether, or may have a thickness in a range from10-290 nm, or some other thickness. The SiN slab 3110 may have athickness in a range from 0.04 to 0.07 μm, or some other thickness. TheSiN transition waveguide 3118, and thus the third layer 3116, may have athickness of 0.5 μm, or a thickness in a range of 0.5 μm plus or minus10%, or some other thickness. The SiN transition waveguide 3118 may havea width in the x direction other than at one or more tapered endsthereof in a range from 1-2 μm, or some other width. The SiO₂ layer 3120immediately beneath the SiN transition waveguide 3118 may have athickness in a range from 0.04-0.07 μm, or some other thickness. The SiNwaveguide 3108, and thus the first layer 3106, may have a thickness in arange from 0.04-0.07 μm, or some other thickness. The SiN waveguide 3108may have a width in the x direction other than at one or more taperedends thereof of 0.6-1 μm, or some other width. The SiO₂ layer 3120immediately beneath the SiN waveguide 3108 may have a thickness in arange from 0.05-0.2 μm, or some other thickness.

The overhead view 3101B illustrates relative x-axis and z-axis alignmentof various w components of the Si PIC 3100 with respect to each otherand includes reference lines 1, 2, 3, and 4. The relative x-axis andz-axis alignment between the Si waveguide 3114, the SiN transitionwaveguide 3118, and the SiN waveguide 3108 and aspects of each of theforegoing waveguides will now be described. As illustrated, the SiNwaveguide 3108 includes a tapered end between reference lines 3 and 4.Although not illustrated in FIG. 31A, the SiN waveguide 3108 may includeanother tapered end opposite the tapered end illustrated in FIG. 31A toadiabatically couple light into a corresponding interposer waveguide orto adiabatically receive light from the interposer waveguide.

The SiN transition waveguide 3118 includes a coupler portion betweenreference lines 1 and 3 at a first end of the SiN transition waveguide3118. The SiN transition waveguide 3118 also includes a tapered endbetween reference lines 3 and 4 opposite the first end. The tapered endof the SiN transition waveguide 3118 is aligned in two orthogonaldirections (e.g., in the x and z directions) with the tapered end of theSiN waveguide 3108 such that the tapered end of the SiN transitionwaveguide 3118 overlaps in the two orthogonal directions and is parallelto the tapered end of the SiN waveguide 3108.

The Si waveguide 3114 includes a tapered end between reference lines 2and 3. The tapered end of the Si waveguide 3114 is aligned in the twoorthogonal direction (e.g., in the x and z directions) with the couplerportion of the SiN transition waveguide 3118 such that the tapered endof the Si waveguide 3114 overlaps in the two orthogonal directions andis parallel to the coupler portion of the SiN transition waveguide 3118.

As illustrated in the overhead view 3101B, the tapered end of the Siwaveguide 3114 may terminate where the tapered end of the SiN waveguide3108 begins, e.g., at reference line 3. Alternately or additionally, aregion in which the tapered ends of the SiN waveguide 3108 and the SiNtransition waveguide 3118 overlap may be referred to as a dual taperregion 3122. The dual taper region 3121 may have a length in the zdirection of at least 20 μm, or at least 30 μm, or some other length.

The Si PIC 3100 may include one or more other components, layers,features, or aspects as described elsewhere herein.

FIG. 31B illustrates first-fourth simulations 3124A-3124C for the Si PIC3100 of FIG. 31A, arranged in accordance with at least one embodimentdescribed herein. Due to the SiN transition waveguide 3118 beingseparated from the Si substrate 3102 by about 1.1 μm in the example ofFIG. 31A, some substrate leakage may occur for light propagating throughthe SiN transition waveguide 3118. However, a total length of the SiNtransition waveguide 3118 in the z direction may be relatively short,such as about 100 μm or less, such that the substrate leakage may berelatively low. On the other hand, the SiN waveguide 3108 may beseparated from the Si substrate 3102 by 1.2 μm or more, or even 1.6 μmor more, such that light propagating through the SiN waveguide 3108 mayexperience little or no substrate leakage, such as about 0.1 dB/cm forthe TE optical mode and about 0.35 dB/cm for the TM optical mode.

The first and second simulations 3124A and 3124B illustrate propagationof the TE and TM optical modes, respectively, from the SiN transitionwaveguide 3118 in the area generally labeled “Layer 1” to the Siwaveguide 3118 in the area generally labeled “Layer 2. The thirdsimulation 3124C includes a graph of transmission efficiency from theSiN transition waveguide 3118 to the SiN waveguide 3108 along thevertical axis as a function of dual taper length along the horizontalaxis in units of μm. The dual taper length refers to the length of thedual taper region 3122. As illustrated in the third simulation 3124C,the transmission efficiency increases with increasing dual taper lengthand is about 90% or higher for both TE and TM optical modes at a dualtaper length of about 20 μm and is about 96% or higher for both TE andTM optical modes at a dual taper length of about 30 μm.

Some Si PICs may include a WDM mux or WDM demux as described elsewhereherein, such as an Echelle grating in a SiN layer of the Si PIC. As usedherein, a SiN layer of a Si PIC refers to a layer of the Si PIC thatincludes SiN, which layer may additionally include other materials suchas SiO₂ in various locations within the SiN layer. In a WDM demuxconfiguration, incoming light received from the WDM demux may be coupledfrom a SiN waveguide through a Si waveguide in a Si layer of the Si PICto a Si/germanium (Ge) based pin detector included in the Si layer ofthe Si PIC. As used herein, a Si layer of a Si PIC refers to a layer ofthe Si PIC that includes Si, which layer may additionally include othermaterials such as SiO₂ in various locations within the Si layer. SomeWDM demuxes have to have a multimode output to allow a flat top shapefor a filter function associated with the WDM demuxes. For example, aSiN-based WDM demux may utilize TE₀₀, TE₀₁, TM₀₀, and TM₀₁ opticalmodes. Some of the SiN-to-Si adiabatic coupler regions described abovemay accommodate single mode light. Such single mode adiabatic couplerregions may reduce effective bandwidth of a WDM demux with a multimodeoutput, since only a single mode may be coupled from the SiN waveguideto the Si waveguide.

Some embodiments described herein may include a multimode SiN-to-Siadiabatic coupler region to accept demultiplexed and/or multimode outputof a WDM demux without reducing effective bandwidth of the WDM demux. Inparticular, FIG. 32 illustrates a multimode SiN-to-Si adiabatic couplerregion 3200 (hereinafter “coupler 3200”), arranged in accordance with atleast one embodiment described herein. The coupler 3200 may beimplemented in any of the Si PICs described herein. Such Si PICs maygenerally include a SiO₂ box, a first layer formed above the SiO₂ boxthat includes a SiN waveguide 3202, and a second layer formed above theSiO₂ box and above or below the first layer and that includes a Siwaveguide 3204.

The SiN waveguide 3202 includes an untapered end portion 3206 and atapered end 3208 that begins where the untapered end portion 3206begins, the untapered end portion 3206 and the tapered end 3208extending in opposite directions. Although not illustrated in FIG. 32,the SiN waveguide 3202 may extend to the left of the untapered endportion 3206. The untapered end portion 3206 may receive a multimodeinput optical signal 3210 such as may be output by a SiN-based WDMdemux.

The Si waveguide 3204 includes an untapered end portion 3212 and atapered end 3214 that begins where the untapered end portion 3212begins, the untapered end portion 3212 and the tapered end 3214extending in opposite directions. The Si waveguide 3204 may extend tothe right of the untapered end portion 3212. The Si waveguide 3204 maybe configured to accept the multimode input optical signal 3210 from theSiN waveguide 3202.

In some embodiments, the untapered end portion 3206 of the SiN waveguide3202 is aligned in two orthogonal directions (e.g., the x and zdirections) with the tapered end 3214 of the Si waveguide 3204 such thatthe untapered end portion 3206 of the SiN waveguide 3202 overlaps in thetwo orthogonal directions and is parallel to the tapered end 3214 of theSi waveguide 3204. Additionally, the tapered end 3208 of the SiNwaveguide 3202 is aligned in the two orthogonal directions with theuntapered end portion 3212 of the Si waveguide 3204 such that thetapered end 3208 of the SiN waveguide 3202 overlaps in the twoorthogonal directions and is parallel to the untapered end portion 3212of the Si waveguide 3204.

A region in which the untapered end portion 3206 of the SiN waveguide3202 and the tapered end 3214 of the Si waveguide 3204 overlap may bereferred to as a first region 3216. A region in which the tapered end3208 of the SiN waveguide 3202 and the untapered end portion 3212 of theSi waveguide 3204 overlap may be referred to as a second region 3218.Lengths of the first region 3216 and the second region 3218 and/or otherparameters associated with the coupler 3200 may be adjusted to optimizethe multimode coupling from the SiN waveguide 3202 to the Si waveguide3204, as illustrated in FIGS. 33A-33D.

FIGS. 33A-33D include various simulations for the coupler 3200 of FIG.32 with various different sets of parameters, arranged in accordancewith at least one embodiment described herein.

FIG. 33A includes a first table 3302 of parameters, a second table 3304of simulated transmission efficiency from the SiN waveguide 3202 to theSi waveguide 3204 of FIG. 32, and simulations 3306A and 3306B. Withcombined reference to FIGS. 32 and 33A, the parameters of FIG. 33A thatare listed in the first table 3302 will now be described. In thisexample, the first region 3216 has a length of 90 μm and the secondregion 3218 has a length of 10 μm. In the first region 3216, the taperedend 3214 of the Si waveguide 3204 has a width that tapers along thelight propagation direction from 0.08 μm to 1.5 μm. In the second region3218, the untapered end portion 3212 of the Si waveguide 3204 has awidth of 1.5 μm. In the first region 3216, the untapered end portion3206 of the SiN waveguide 3202 has a width of 2 μm. In the second region3218, the tapered end 3208 of the SiN waveguide 3202 has a width thattapers along the light propagation direction from 2 μm to 0.2 μm.

The second table 3304 includes simulated transmission efficiency for theTE₀₀, TE₀₁, TM₀₀, and TM₀₁ optical modes associated with the parameterslisted in the first table 3302.

The simulations 3306A and 3306B include graphs of transmissionefficiency in the coupler 3200 along the vertical axis as a function ofSi taper length along the horizontal axis in units of μm for the TENoptical mode (simulation 3306A) and the TM₀₁ optical mode (simulation3306B) for five different wavelength channels. The Si taper lengthrefers to the length of the first region 3216. In the simulations 3306Aand 3306B, all parameters other than the length of the first region 3216are assumed to be the parameters provided in the first table 3302.

FIG. 33B includes simulations 3306C and 3306D that use the sameparameters as the simulations 3306A and 3306B of FIG. 33A except thatthe untapered end portion 3206 of the SiN waveguide 3202 has a width of1.5 μm in the first region 3216 and the tapered end 3208 of the SiNwaveguide 3202 tapers from 1.5 μm to 0.2 μm in the second region 3218.

FIG. 33C includes simulations 3306E and 3306F that use the sameparameters as the simulations 3306C and 3306D of FIG. 33B except thatthe tapered end 3214 of the Si waveguide 3204 tapers from 0.08 μm to 1μm in the first region 3216 and the untapered end portion 3212 of the Siwaveguide 3204 has a width of 1 μm in the second region 3218. Asillustrated in the simulations 3306E and 3306F, at a Si taper length (orfirst region 3216 length) of 90 μm, the TE₀₁ optical mode has atransmission efficiency of about 0.96 for all five wavelength channelsand the TM₀₁ optical mode has a transmission efficiency between about0.92-0.96 depending on the wavelength channel.

FIG. 33D includes simulations 3306G and 3306H that are similar to thesimulations 3306A-3306E described above, except using parameters listedin table 3308. As illustrated in the simulations 3306G and 3306H, at aSi taper length (or first region 3216 length) of 100 μm, the T₀₁ opticalmode has a transmission efficiency of between about 0.95-0.97 dependingon the wavelength channel and the TM₀₁ optical mode has a transmissionefficiency between about 0.92-0.95 depending on the wavelength channel.

One or more of the WDM components described herein may have apolarization-dependent filter function. In these and other embodiments,one or more of the Si PICs described herein may further include one ormore Si PIC polarization splitters or combiners (hereinafter“polarization splitter” or “polarization splitters’). The Si PIC mayadditionally include two polarization-specific WDM components, each ofwhich has an input coupled to a different output of the polarizationsplitter. One of the polarization-specific WDM components may beoptimized for TE polarization and the other may be optimized for TMpolarization. Alternatively, each of the polarization-specific WDMcomponents may be optimized for the same polarization and the SI PIC mayadditionally include a polarization rotator coupled between one of thetwo outputs of the polarization splitter and the input of one of thepolarization-specific WDM components. The polarization rotator mayinclude a Si PIC polarization rotator integrally formed in the Si PIC.

FIGS. 34A and 34B illustrate embodiments of a demultiplexer system 3400Aand 3400B (collectively “demultiplexer systems 3400”), arranged inaccordance with at least one embodiment described herein. Some or all ofthe demultiplexer systems 3400 may be implemented in a Si PIC, such asthe Si PICs described above. The demultiplexer systems 3400 each includea Si PIC polarization splitter or combiner 3402 (hereinafter“polarization splitter 3402”) a first WDM demux 3404, a second WDM demux3406A or 3406B (generically “second WDM demux 3406”), firstopto-electrical transducers 3408, second opto-electrical transducers3410, and adders 3412 (only one of which is illustrated for simplicity).Additional adders 3412 are denoted by ellipses in each of FIGS. 34A and34B. The demultiplexer system 3400B of FIG. 34B may additionally includea polarization rotator 3414.

The polarization splitter 3402 in each of the demultiplexer systems 3400includes an input 3402A and first and second outputs 3402B and 3402Cexcepted when implemented as a combiner, in which case the inputs andoutputs may be reversed. As described in more detail below, thepolarization splitter 3402 may generally include first and second SiNwaveguides formed in a corresponding layer of a Si PIC and a Siwaveguide with two tapered ends formed in another layer of the Si PICabove or below the layer in which the first and second SiN waveguidesare formed. In some embodiments, the first and second WDM demuxes 3404and 3406 may be formed in the same layer of the Si PIC as the first andsecond SiN waveguides of the polarization splitter 3402, as describedelsewhere herein.

The input 3402A may include a first end of the first SiN waveguide, thefirst output 3402B may include a second end of the first SiN waveguide,and the second output 3402C may include a second end of the second SiNwaveguide. On the input, the polarization splitter 3402 may receive aninput beam 3415 including an N-channel optical signal (e.g., amultiplexed optical signal with N wavelength channels λ₁, λ₂, λ₃, . . ., λ_(n)) with two orthogonal polarizations, e.g., TE polarization and TMpolarization. The input beam 3415 may be split according topolarization, with the TE polarization generally being outputted fromthe first output 3402B and the TM polarization generally being outputtedfrom the second output 3402C.

Each of the first and second WDM demuxes 3404 and 3406 may be optimizedfor and/or specific to one of the two polarizations depending on thepolarization of light that is input to the first or second WDM demux3404 or 3406. For example, the first WDM demux 3404 in s FIGS. 34A and34B and the second WDM demux 3406B in FIG. 34B may be optimized for orspecific to the TE polarization. The second WDM demux 3406A in FIG. 34Amay be optimized for or specific to the TM polarization. In these andother embodiments, each of the first and second WDM demuxes 3404 and3406 may include an Echelle grating with a polarization-dependent filterfunction.

The first WDM demux 3404 includes an input 3416 optically coupled to thefirst output 3402B of the polarization splitter 3402. Analogously, thesecond WDM demux 3406A or 3406B respectively includes an input 3418 or3420 optically coupled to the second output 3402C or to the polarizationsplitter 3402.

The first WDM demux 3404 additionally includes outputs 3422 opticallycoupled to the first opto-electrical transducers 3408. Analogously, thesecond WDM demux 3406A or 3406B respectively includes outputs 3424 or3426 optically coupled to the second opto-electrical transducers 3410.The first opto-electrical transducers 3408 and the secondopto-electrical transducers 3410 may each include at least N PN diodes,avalanche photodiodes (APDs), or other suitable optical receivers.

The adders 3412 are electrically coupled to outputs of the first andsecond opto-electrical transducers 3408 and 3410, where each of theadders 3412 is electrically coupled to an output of a corresponding oneof the first opto-electrical transducers 3408 and to an output of acorresponding one of the second opto-electrical transducers 3410. Inparticular, for i=1 to N, an ith one of the adders 3412 may beelectrically coupled to an ith one of the first opto-electricaltransducers 3408 and to an ith one of the second opto-electricaltransducers 3410 to sum an electrical output of the ith one of the firstopto-electrical transducers 3408 with an electrical output of the ithone of the second opto-electrical transducers 3410 to generate an ithcombined electrical output 3428.

In FIGS. 34A and 34B, in operation, the first WDM demux 3404 receivesthe TE polarization of the input beam 3415 and demultiplexes it into Ndistinct wavelength channels λ₁, λ₂, λ₃, . . . , λ_(N) that are outputto the first opto-electrical transducers 3408. The first opto-electricaltransducers 3408 each output an electrical signal representative of acorresponding one of the N distinct wavelength channels received at thecorresponding one of the first opto-electrical transducers 3408.

In FIG. 34A, in operation, the second WDM demux 3406A receives the TMpolarization of the N-channel optical signal from the second output3402C of the polarization splitter 3402 and demultiplexes it into Ndistinct wavelength channels that are output to the secondopto-electrical transducers 3410. The second opto-electrical transducers3410 each output an electrical signal representative of a correspondingone of the N distinct wavelength channels received at the correspondingone of the second opto-electrical transducers 3410.

In FIG. 34B, in operation, the polarization rotator 3414 rotates apolarization of the TM polarization received from the second output3402C of the polarization splitter 3402 from the TM polarization to theTE polarization. In this and other embodiments, the polarization rotator3414 may include a TM-to-TE polarization rotator. More generally, thepolarization rotator 3414 may rotate the polarization from a first (orsecond) polarization to an orthogonal second (or first) polarization.The second WDM demux 3406A then receives the polarization-rotated signalfrom the polarization rotator 3414 and demultiplexes it into N distinctwavelength channels that are output to the second opto-electricaltransducers 3410. The second opto-electrical transducers 3410 eachoutput an electrical signal representative of a corresponding one of theN distinct wavelength channels received at the corresponding one of thesecond opto-electrical transducers 3410.

In both FIGS. 34A and 34B, the adders 3412 then combine the appropriateoutputs from the first and second opto-electrical transducers 3408 and3410 to generate an ith combined electrical signal 3428 that isrepresentative of the ith wavelength channel from the input beam 3415received at the input 3402A of the polarization splitter 3402. Inparticular, a first (or second, or third, or Nth) one of the ithcombined electrical signals 3428 includes a sum of the electrical outputof a first (or second, or third, or Nth) one of the firstelectro-optical transducers 3408 that is representative of a first (orsecond, or third, or Nth) one of the N distinct wavelength channelsoutput by the first WDM demux 3404 and the electrical output of a first(or second, or third, or Nth) one of the second electro-opticaltransducers 3410 that is representative of a first (or second, or third,or Nth) one of the N distinct wavelength channels output by the secondWDM demux 3406A.

By splitting the TE polarization from the TM polarization,demultiplexing each separately from the other, and then addingcorresponding channels with the adders 3412, the demultiplexer systems3400 of FIGS. 34A and 34B may eliminate or at least significantly reducechannel cross-talk that arises in WDM demuxes withpolarization-dependent filter functions.

Various considerations and parameters associated with a Si PICpolarization splitter, such as the polarization splitter 3402, will nowbe discussed with respect to FIGS. 35-37, followed by a discussion ofvarious example Si PIC polarization splitters with respect to FIGS.38A-38C.

FIG. 35 is a graphical representation 3500 of a simulation of effectiveindex as a function of Si waveguide width for TE and TM polarizations inSi and SiN waveguides of an adiabatic coupler region, arranged inaccordance with at least one embodiment described herein. It can be seenfrom curves 3506 and 3508 of FIG. 35 that the effective index for TE andTM polarizations in the SiN waveguide does not vary with Si waveguidewidth and has a value of about 1.7. It can be seen from curves 3502 and3504 of FIG. 35 that the effective index for TE polarization in the Siwaveguide (see curve 3502) is less than 1.7 in the region from 130 nm to180 nm (or 0.13 μm to 0.18 μm) and increases across this region, and theeffective index for TM polarization in the Si waveguide (see curve 3504)is greater than 1.7 in the region from 130 nm to 180 nm and increasesacross this region. As such, TE and TM polarizations will necessarilyhave different coupling efficiencies in the adiabatic coupler region ifa tip width of a tapered end of the Si waveguide is between 130 nm to180 nm. Differences between the TE and TM coupling efficiencies forvarious tip widths in the 130 nm to 180 nm range are illustrated inFIGS. 36 and 37.

The Si waveguide width at which the effective index for TM polarizationin the Si waveguide (curve 3504) crosses over the effective index for TMpolarization in the SiN waveguide (curve 3508) may be referred to hereinas a “TM maximum taper width”, and is about 100 nm in FIG. 35. If a tipwidth of a tapered end of the Si waveguide is greater than the TMmaximum taper width, it can be seen from FIG. 35 that adiabatic couplingwith high efficiency of the TM polarization between the Si waveguide andthe SiN waveguide may be prevented relative to coupling efficiency ofthe TE polarization. Analogously, the Si waveguide width at which theeffective index for TE polarization in the Si waveguide (curve 3502)crosses over the effective index for TE polarization in the SiNwaveguide (curve 3506) may be referred to herein as a “TE maximum taperwidth”, and is about 180 nm in FIG. 35. If a tip width of a tapered endof the Si waveguide is less than the TE maximum taper width, it can beseen from FIG. 35 that adiabatic coupling with high efficiency of the TEpolarization between the Si waveguide and the SiN waveguide may bepermitted relative to coupling efficiency of the TM polarization.

FIG. 36 is a graphical representation 3600 of a simulation of TE and TMpolarization coupling efficiency as a function of Si waveguide taperlength for a Si waveguide tip width of 180 nm and 150 nm, arranged inaccordance with at least one embodiment described herein. In particular,for a tip width of 180 nm, curve 3602 represents TE coupling efficiencywhile curve 3604 represents TM coupling efficiency. Analogously, for atip width of 150 nm, curve 3606 represents TE coupling efficiency whilecurve 3608 represents TM coupling efficiency. It can be seen from curves3602, 3604, 3606, and 3608 that at both tip widths, the TE polarization(curves 3602 and 3606) has a much better coupling efficiency than the TMpolarization (curves 3604 and 3608). Curves 3602 and 3606 tend toindicate that for tip widths equal to or greater than 180 nm, TEcoupling may be below 90%. Curves 3604 and 3608 tend to indicate thatfor tip widths less than or equal to 150 nm, TM coupling may be greaterthan 10%.

FIG. 37 is a graphical representation 3700 of a simulation of TE and TMpolarization coupling efficiency as a function of Si waveguide taperlength for a Si waveguide tip width of 160 nm for three differentwavelength channels at 1.35 μm, 1.31 μm, and 1.27 μm, arranged inaccordance with at least one embodiment described herein. The tip widthof 160 nm is selected as a compromise between 150 nm (below which TMcoupling may be greater than 10%) and 180 nm (above which TE couplingefficiency may be less than 90%). For a tip width of 160 nm and a 1.35μm wavelength channel, curve 3702A represents TE coupling efficiencywhile curve 3702B represents TM coupling efficiency. Analogously, for atip width of 160 nm and a 1.31 μm wavelength channel, curve 3704Arepresents TE coupling efficiency while curve 3704B represents TMcoupling efficiency. Analogously, for a tip width of 160 nm and a 1.27μm wavelength channel, curve 3706A represents TE coupling efficiencywhile curve 3706B represents TM coupling efficiency. It can be seen fromcurves 3702A, 3702B, 3704A, 3704B, 3706A, and 3706B that at all threewavelength channels, the TE polarization (curves 3702A, 3704A, and3706A) has a much better coupling efficiency than the TM polarization(curves 3702B, 3704B, and 3706B).

FIG. 37 additionally includes a table 3708 with various TE and TMpolarization coupling efficiency values for the three wavelengthchannels at 1.35 μm, 1.31 μm, and 1.27 μm where the Si waveguide taperlength is about 200 μm. For each wavelength channel, a ratio of TEpolarization coupling efficiency to TM polarization coupling efficiencyis also provided in units of decibels (dB).

The simulations of FIGS. 35-37 indicate that, at least in someembodiments, an adiabatic coupler region that includes a Si waveguidewith a tip width between 130 nm to 180 nm, or between 150 nm to 180 nm,or at about 160 nm, may be used to selectively couple most of the TEpolarization from the Si waveguide to the SiN waveguide (or vice versa)without coupling most of the TM polarization from the Si waveguide tothe SiN waveguide (or vice versa). Two or more such adiabatic couplerregions may be combined as described in more detail with respect toFIGS. 38A-38C to form a Si PIC polarization splitter or combiner, suchas the polarization splitter 3402 discussed above.

FIGS. 38A-38C illustrate example Si PIC polarization splitters orcombiners 3800A, 3800B, and 3800C (hereinafter collectively“polarization splitters 3800”), arranged in accordance with at least oneembodiment described herein. The polarization splitters 3800 may includeor correspond to the polarization splitter 3402 of FIGS. 34A and 34B andmay be implemented in the demultiplexer systems 3400 of FIGS. 34A and34B and/or in other systems or devices.

FIGS. 38A-38C each include an overhead view of the polarization splitter3800A, 3800B, or 3800C. The overhead views of FIGS. 38A-38C includeoutlines or footprints of various components of the polarizationsplitters 3800 at different levels in a material stack up of thepolarization splitters 3800 that may not necessarily be visible whenviewed from above, but are shown as outlines or footprints to illustratex and z alignment of the various components with respect to each other.

Each of the polarization splitters 3800 includes a first SiN waveguide3802, a second SiN waveguide 3804 spaced apart from the first SiNwaveguide 3802, and a Si waveguide 3806. The first and second SiNwaveguides 3802 and 3804 may be formed in a first layer of a Si PIC,such as any of the first layers with SiN waveguides described herein.The Si waveguide 3806 may be formed in a second layer of the Si PIC thatis above or below the first layer of the Si PIC, such as any of thesecond layers with Si waveguides described herein.

The first SiN waveguide 3802 includes a coupler portion 3808, the secondSiN waveguide 3804 includes a coupler portion 3810, and the Si waveguide3806 includes a first tapered end 3812 and a second tapered end 3814.The first tapered end 3812 is aligned in two orthogonal directions(e.g., x and z) with the coupler portion 3808 of the first SiN waveguide3802 such that the first tapered end 3812 overlaps in the two orthogonaldirections and is parallel to the coupler portion 3808 of the first SiNwaveguide 3802. The first tapered end 3812 and the coupler portion 3808of the first SiN waveguide 3802 may generally form a first adiabaticcoupler region 3816. Analogously, the second tapered end 3814 is alignedin two orthogonal directions (e.g., x and z) with the coupler portion3810 of the second SiN waveguide 3804 such that the second tapered end3814 overlaps in the two orthogonal directions and is parallel to thecoupler portion 3810 of the second SiN waveguide 3804. The secondtapered end 3814 and the coupler portion 3810 of the second SiNwaveguide 3804 may generally form a second adiabatic coupler region3818.

Each of the first and second tapered ends 3812 and 3814 of the Siwaveguide 3806 may be configured to adiabatically couple most of a firstpolarization (e.g., TE polarization) of an input beam 3820 between acorresponding one of the first and second tapered ends 3812 and 3814 ofthe Si waveguide 3806 and a corresponding one of the first and secondSiN waveguides 3802 and 3804 and to prevent most of a secondpolarization (e.g., TM polarization) of the input beam 3820 that isorthogonal to the first polarization from being adiabatically coupledbetween the corresponding one of the first and second tapered ends 3812and 3814 and the corresponding one of the first and second SiNwaveguides 3802 and 3804. The foregoing may be accomplished by providingeach of the first and second tapered ends 3812 and 3814 of the Siwaveguide 3806 with an appropriate tip width that generallydiscriminates between the first and second polarizations.

In more detail, the first tapered end 3812 of the Si waveguide 3806 mayhave a tip width configured to adiabatically couple most of the firstpolarization from the first SiN waveguide 3802 through the first taperedend 3812 to the Si waveguide 3806 and to prevent most of the secondpolarization from entering the Si waveguide 3806. For example, the firsttapered end 3812 may have a tip width in a range between 130 nm and 180nm, or in a range between 150 nm and 180 nm, or a tip width of about 160nm. Analogously, the second tapered end 3814 of the Si waveguide 3804may have a tip width configured to adiabatically couple most of aportion of the first polarization propagating through the Si waveguide3806 from the Si waveguide 3806 through the second tapered end 3814 tothe second SiN waveguide 3804 and to prevent most of a portion of thesecond polarization propagating through the Si waveguide 3806 fromentering the second SiN waveguide 3804. For example, the second taperedend 3814 may have a tip width in a range between 130 nm and 180 nm, orin a range between 150 nm and 180 nm, or a tip width of about 160 nm.Accordingly, and consistent with FIGS. 35-37, a tip width of the firstand second tapered ends 3812 and 3814 may be configured to selectivelycouple most of the first polarization of the input beam 3820 from thefirst SiN waveguide 3802 to the second SiN waveguide 3804 withoutcoupling most of the second polarization from the first SiN waveguide3802 to the second SiN waveguide 3804.

In the example of FIG. 38A, the Si waveguide 3806 may have a taperlength of 200 nm (e.g., each of the first and second tapered ends 3812and 3814 may be 200 nm long in a light propagation direction) and eachof the first and second tapered ends 3812 and 3814 may have a tip widthof 150 nm. Alternatively or additionally, the first and second SiNwaveguides 3802 and 3804 may each have a width of 1 μm and each of thefirst and second tapered ends 3812 and 3814 may have a maximum width of320 nm. In this example, and for a 1.31 μm wavelength channel, each ofthe first and second adiabatic coupler regions 3816 and 3818 mayadiabatically couple about 98% of the TE polarization and about 10% ofthe TM polarization from one waveguide to the next (e.g., from the firstSiN waveguide 3802 to the Si waveguide 3806 or from the Si waveguide3806 to the second SiN waveguide 3804) and may prevent about 2% of theTE polarization and about 90% of the TM polarization from beingadiabatically coupled from one waveguide to the next. As a result, inFIG. 38A, an output beam 3822 from an end 3824 of the first SiNwaveguide 3802 may include about 2% of the TE polarization and about 90%of the TM polarization of the input beam 3820. Because an output beam3826 from an end 3828 of the second SiN waveguide 3804 passes throughboth the first and the second adiabatic coupler regions 3816 and 3818,the output beam 3826 may include about 96% of the TE polarization andabout 1% of the TM polarization of the input beam 3820.

In FIGS. 38B and 38C, each of the polarization splitters 3800B and 3800Cadditionally includes a third adiabatic coupler region 3830 or 3832 toimprove a split ratio of the TE and TM polarizations in the output beam3822 from the end 3824 of the first SiN waveguide 3802. The thirdadiabatic coupler region 3830 or 3832 may be made up of a second couplerportion 3834 of the first SiN waveguide 3802 and a tapered end 3836 or3838 of a second Si waveguide 3840 or 3842. The second Si waveguide 3840or 3842 may be formed in the same layer of the Si PIC as the Siwaveguide 3806, or in a different layer of the Si PIC than the Siwaveguide 3806.

Alternatively or additionally, the first SiN waveguide 3802 may includea tapered end 3844 upstream of the coupler portion 3808. In an exampleembodiment, the tapered end 3844 of the first SiN waveguide 3802 has ataper length (e.g., a length in the z direction) of about 50 μm. SiNwaveguides in Si PICs according to some embodiments described herein maygenerally have a width (e.g., in the x direction) of about 0.7 μm orless, and may be referred to as standard SiN waveguides. In comparison,SiN waveguides in Si PIC polarization splitters such as the polarizationsplitters 3402 and 3800 described herein may have different widths thanthe standard SiN waveguides, e.g., widths of about 1 μm, and may bereferred to as polarization splitter SiN waveguides. The tapered end 844of the first SiN waveguide 3802 may serve as a transition from astandard SiN waveguide to the first SiN waveguide 3802 which is apolarization splitter SiN waveguide.

The tapered end 3836 or 3838 of the second Si waveguide 3840 or 3842 isaligned in two orthogonal directions (e.g., x and z) with the secondcoupler portion 3834 of the first SiN waveguide 3802 such that thetapered end 3836 or 3838 of the second Si waveguide 3840 or 3842overlaps in the two orthogonal directions and is parallel to the secondcoupler portion 3834 of the first SiN waveguide 3802. The second Siwaveguide 3840 in FIG. 38B generally includes an S shape, whereas thesecond Si waveguide 3842 in FIG. 38C generally includes a U shape. Othershapes may alternatively be implemented. In some embodiments, each ofthe second Si waveguides 3840 and 3842 includes a second tapered end3846 or 3848 opposite the tapered end 3836 or 3838. In otherembodiments, each of the second Si waveguides 3840 and 3842 terminatesat a germanium (Ge) PIN detector rather than with the second tapered end3846 or 3848.

Each of the tapered ends 3836 or 3838 of the second Si waveguide 3840 or3842 may have an appropriate tip width to generally discriminate betweenthe first and second polarizations. In more detail, the tapered end 3836or 3838 of the second Si waveguide 3840 or 3842 may have a tip widthconfigured to adiabatically couple most of the first polarization fromthe first SiN waveguide 3802 through the tapered end 3836 or 3838 to thesecond Si waveguide 3840 or 3842 and to prevent most of the secondpolarization from entering the second Si waveguide 3840 or 3842. Forexample, the tapered end 3836 or 3838 may have a tip width in a rangebetween 130 nm and 180 nm, or in a range between 150 nm and 180 nm, or atip width of about 160 nm. In some embodiments, the second tapered end3846 or 3848 of the second Si waveguide 3840 or 3842 may similarly havea tip width in a range between 130 nm and 180 nm, or in a range between150 nm and 180 nm, or a tip width of about 160 nm.

In the example of FIGS. 38B and 38C, the Si waveguide 3806 may have ataper length of 200 nm and each of the first and second tapered ends3812 and 3814 may have a tip width of 160 nm. Alternatively oradditionally, the second Si waveguide 3840 or 3842 may also have a taperlength of 200 nm, each of the tapered end 3836 or 3838 and the secondtapered end 3846 or 3848 may have a tip width of 160 nm, and the firstand second SiN waveguides 3802 and 3804 may have a width of 1 μm.Alternatively or additionally, the first and second tapered ends 3812and 3814 of the Si waveguide 3806, the tapered end 3836 or 3838 of thesecond Si waveguide 3840 or 3842, and/or the second tapered end 3846 or3848 of the second Si waveguide 3840 or 3842 may have a maximum width of320 nm. In this example, and for a 1.31 μm wavelength channel, each ofthe first, second, and third adiabatic coupler regions 3816, 3818, and3830 or 3832 may adiabatically couple about 97.7% of the TE polarizationand about 6.7% of the TM polarization from one waveguide to the next(e.g., from the first SiN waveguide 3802 to the Si waveguide 3806, fromthe Si waveguide 3806 to the second SiN waveguide 3804, or from thefirst SiN waveguide 3802 to the second Si waveguide 3840 or 3842) andmay prevent about 2.3% of the TE polarization and about 93.3% of the TMpolarization from being adiabatically coupled from one waveguide to thenext. As a result, and because the output beam 3822 passes through boththe first and third adiabatic coupler regions 3816 and 3830, the outputbeam 3822 from the end 3824 of the first SiN waveguide 3802 may includeabout 0.05% of the TE polarization and about 87% of the TM polarizationof the input beam 3820. Also, because the output beam 3826 from the end3828 of the second SiN waveguide 3804 passes through both the first andthe second adiabatic coupler regions 3816 and 3818, the output beam 3826may include about 95% of the TE polarization and about 0.5% of the TMpolarization of the input beam 3820. As such, in the example of FIGS.38B and 38C, the ratio of TM/TE in the output beam 3822 may be about 32dB, and the ratio of TE/TM in the output beam 3826 may be about 23 dB.More generally, the tip widths of one or both of the first and secondtapered ends 3812, 3814 of the Si waveguide 3806 may be configured topass at least 80% of the TM polarization through the first SiN waveguide3802 and to adiabatically pass at least 90% of the TE polarization fromthe first SiN waveguide 3802 to the second SiN waveguide 3804.

Alternatively or additionally, one or more of the polarization splitters3800 may be implemented as a polarization combiner. In these and otherembodiments, a TM input beam may be received at the end 3824 of thefirst SiN waveguide 3802 and a TE input beam may be received at the end3828 of the second SiN waveguide 3804. In this example, the secondtapered end 3814 of the Si waveguide 3806 may have a tip width in arange between 130 nm and 180 nm, or even less than 130 nm. The firsttapered end 3812 of the Si waveguide 3806 may have a tip width in arange between 130 nm and 180 nm, or in a range between 150 nm and 180nm, or a tip width of about 160 nm. The TM input beam may propagatethrough the first SiN waveguide 3802 from right to left. The TE inputbeam may propagate through the second SiN waveguide 3804 from right toleft and may be adiabatically coupled through the adiabatic couplerregion 3818 into the Si waveguide 3806 and through the adiabatic couplerregion 3816 into the first SiN waveguide 3802 where it is combined withthe TM input.

FIGS. 39A and 39B include side views that depict alignment andattachment of a high index glass interposer 3900 (hereinafter“interposer 3900”) and the Si PIC 1700 of FIG. 17, arranged inaccordance with at least one embodiment described herein. The interposer3900 includes a high index glass waveguide block 3902 and one or moreinterposer waveguides 3904. The interposer waveguides 3904 may includehigh index glass waveguides that may be written into the high indexglass waveguide block 3902, e.g., by ion exchange method or UV laserwriting or other suitable index altering radiation or process.

As illustrated in FIG. 39A, the interposer 3900 is aligned to the etchedwindow 1702 of the Si PIC 1700 with the interposer cores 3904 generallyaligned in the x and z directions with the SiN waveguides 1712 of the SiPIC 1700 in the manner described above to form adiabatic couplerregions. The etched window 1702 may be at least partially filled withthe epoxy underfill 1902. As illustrated in FIG. 39A, the interposer3900 may then be moved towards the Si PIC 1700 (or vice versa) asindicated by the arrow 3906 until the interposer cores 3904 are indirect or at least close contact with the SiN waveguides 1712 of the SiPIC 1700.

In the illustrated embodiment, the high index glass waveguide block 3902defines one or more holes or grooves 3908 that extend vertically from abottom surface of the high index glass waveguide block 3902, e.g., inthe positive y direction. Each of the holes or grooves 3908 may have aheight (e.g., in the y direction) of 15 μm to 20 μm, or some otherheight. Each of the holes or grooves may extend a length (e.g., in the zdirection) of a portion of the interposer 3900 configured to be receivedwithin the etched window 1702. The portion of the interposer 3900configured to be received within the etched window 1702 may be between 2mm to 3 mm in length in some embodiments. Alternatively or additionally,a width in the x direction of the interposer 3900 may be about 1.5 mm insome embodiments.

When the interposer 3900 is inserted into the etched window 1702 of theSi PIC 1700, the interposer 3900 may be pressed sufficiently tightagainst the Si PIC 1700 to at least partially displace the epoxyunderfill 902 and thin it out so there is relatively little epoxyunderfill 1902 between the interposer waveguides 3904 and the SiNwaveguides 1712. For instance, a thickness (e.g., in the y direction) ofthe epoxy underfill 1902 in the attached configuration of FIG. 39B maybe less than 1 μm. The displaced epoxy underfill 1902 may at leastpartially fill the holes 3908 to achieve good adhesion of the interposer3900 to the Si PIC 1700.

FIG. 40A includes an upside down perspective view of another high indexglass interposer 4000 (hereinafter “interposer 4000”), arranged inaccordance with at least one embodiment described herein. The interposer4000 includes a high index glass waveguide block 4002 and one or moreinterposer waveguides 4004. The interposer waveguides 4004 may includehigh index glass waveguides that may be written into the high indexglass waveguide block 4002, e.g., by ion exchange, UV laser writing, orother suitable index altering radiation or process. The interposer 4000additionally defines v-grooves 4006 longitudinally adjacent to theinterposer waveugides 4004.

FIG. 40B includes a perspective view of the interposer 4000adiabatically coupled to a Si PIC 4008, arranged in accordance with atleast one embodiment described herein. The interposer 4000 isillustrated in FIG. 40B as being transparent to allow the interposerwaveguides 4004 and the v-grooves 4006 generally on a bottom surface ofthe interposer 4000 to be perceived. As can be seen from FIG. 40B, theinterposer waveguides 4004 are generally disposed within an etchedwindow 4010 defined through one or more dielectric layers of the Si PICabove a first layer of the Si PIC 4008. The first layer may include oneor more SiN waveguides to which the interposer waveguides 4004 areadiabatically coupled within the etched window 4010.

FIG. 40B additionally illustrates optical fibers 4012 to which theinterposer waveguides 4004 may be optically coupled. In particular, endsof the optical fibers may be stripped of a jacket and/or waveguidecladding such that optical fiber cores of the optical fibers 4012 arepositioned within the v-grooves 4006. Insofar as the v-grooves 4006 maygenerally be optically aligned to the interposer waveguides 4004,positioning the optical fibers 4012 such that their optical fiber coresare positioned within the v-grooves 4006 may generally optically aligneach of the optical fibers 4012 to a corresponding one of the interposerwaveguides 4004.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A coupled system comprising; a first waveguidehaving a first refractive index n1 and a tapered end; at least onesecond waveguide, each having a second refractive index n2; aninterposer comprising a third waveguide having a third refractive indexn3 and a coupler portion, wherein: the tapered end of the firstwaveguide is adiabatically coupled to a coupler portion of one of the atleast one second waveguide; a tapered end of one of the at least onesecond waveguide is adiabatically coupled to the coupler portion of thethird waveguide of the interposer; n1>n2>n3; and the coupled system isconfigured to adiabatically couple light between the first waveguide andthe at least one second waveguide and between the at least one secondwaveguide and the third waveguide.
 2. The coupled system of claim 1wherein: the first waveguide has a first optical mode size; each of theat least one second waveguide has a second optical mode size; the thirdwaveguide of the interposer has a third optical mode size; the firstoptical mode size is substantially smaller than the second optical modesize; and the second optical mode size is substantially smaller than thethird optical mode size.
 3. The coupled system of claim 2, wherein thethird optical mode size of the third waveguide of the interposer issubstantially similar to the mode size of a standard single mode opticalfiber, thereby providing for efficient optical coupling of light fromthe first waveguide to single mode optical fiber.
 4. The coupled systemof claim 1, wherein the first waveguide and at the least one secondwaveguide are in a silicon (Si) photonic system and fabricated in a sameSi photonic integrated circuit process and wherein the third waveguideof the interposer is formed in a different material system and attachedseparately to the Si photonic integrated circuit.
 5. The coupled systemof claim 1, wherein the first waveguide comprises a silicon (Si) coreand silicon dioxide (SiO₂) cladding such that the first waveguidecomprises a Si waveguide and each of the at least one second waveguidecomprises a silicon nitride (SiN) core and SiO₂ cladding such that theat least one second waveguide comprises a SiN waveguide.
 6. The coupledsystem of claim 5, wherein the third waveguide of the interposercomprises a polymer.
 7. The coupled system of claim 5, wherein the thirdwaveguide of the interposer comprises a high index glass waveguide. 8.The coupled system of claim 1, wherein a value of the first refractiveindex n1 is in a range of 3 to 3.5 and a value of the second refractiveindex n2 is in a range of 1.8 to 2.2.
 9. The coupled system of claim 8,wherein a value of the third refractive index of the interposer is in arange of 1.49 to 1.6.
 10. The coupled system of claim 5, wherein a widthof the tapered end of the Si waveguide tapers from a first width in arange from 300-330 nanometer (nm) to a tip width of about 80 nm andwherein a width of the tapered end of the SiN waveguide tapers from afirst width in a range from 600 nm to 1000 nm to a tip width in a rangefrom 170 nm to 230 nm.
 11. The coupled system of claim 1, furthercomprising: the plurality of first waveguides that includes the firstwaveguide, each having the first refractive index n1, a first end, and atapered end opposite the first end; a plurality of second waveguidesthat includes the at least one second waveguide; a plurality of thirdwaveguides included in the interposer, each having the third refractiveindex n3 and a coupler portion, wherein: the tapered end of each of theplurality of first waveguides is adiabatically coupled to the couplerportion of a corresponding one of the plurality of second waveguides;the tapered end of each of the plurality of second waveguides isadiabatically coupled to the coupler portion of a corresponding one ofthe plurality of third waveguides; the interposer comprises a high indexglass waveguide block within which the plurality of third waveguides areformed; the plurality of third waveguides include a plurality of receiveinterposer waveguides and a plurality of transmit interposer waveguides;at an input/output surface of the high index glass waveguide block thatis configured to be coupled to an optical fiber end connector, ends ofthe plurality of receive interposer waveguides and the plurality oftransmit interposer waveguides have a double-decker arrangementconfigured to match an arrangement of receive optical fibers andtransmit optical fibers to which the optical fiber end connector iscoupled.
 12. The coupled system of claim 1, further comprising: asemiconductor laser; a first lens positioned in an optical path betweenthe semiconductor laser and an input end of the third waveguide of theinterposer; an optical isolator positioned in the optical path after thefirst lens; and a second lens positioned in the optical path after theoptical isolator, wherein the coupled system is configured toadiabatically couple light emitted by the semiconductor laser andreceived in the third waveguide of the interposer from the thirdwaveguide to the at least one second waveguide and from the at least onesecond waveguide to the first waveguide.
 13. The coupled system of claim1, wherein: the coupled system further comprises a plurality of secondwaveguides that includes the at least one second waveguide, a pluralityof second input waveguides and a second output waveguide, each havingthe second refractive index n2; a coupler portion of the second outputwaveguide is adiabatically coupled to the tapered end of the firstwaveguide; the coupled system further comprises: a wavelength divisionmultiplexer (WDM mux) with a plurality of inputs each coupled to acorresponding one of the plurality of second input waveguides and anoutput coupled to the second output waveguide; a plurality of thirdwaveguides included in the interposer, each having the third refractiveindex n3 and a coupler portion, wherein the coupler portion of each ofthe plurality of third waveguides is adiabatically coupled to a taperedend of a corresponding one of the plurality of second input waveguides;a plurality of semiconductor lasers; a plurality of first lenses, eachpositioned in a corresponding optical path between a corresponding oneof the plurality of semiconductor lasers and an input end of acorresponding one of the plurality of third waveguides; a plurality ofoptical isolators, each positioned in the corresponding optical pathafter the corresponding one of the plurality of first lenses; and aplurality of second lenses, each positioned in the corresponding opticalpath after the corresponding one of the plurality of optical isolators.14. The coupled system of claim 1, wherein: the coupled system furthercomprises a plurality of second waveguides, each having the secondrefractive index n2; a tapered end of a first one of the plurality ofsecond waveguides is adiabatically coupled to the coupler portion of thethird waveguide; the coupled system further comprises: an arrayedwaveguide grating (AWG) with a first input or output coupled to thefirst one of the plurality of second waveguides and with a plurality ofsecond outputs or inputs each coupled to a corresponding one of othersof the plurality of second waveguides; and a plurality of firstwaveguides that includes the first waveguide, each having the firstrefractive index n1 and a tapered end, wherein the tapered end of eachof the plurality of first waveguides is adiabatically coupled to acoupler portion of a corresponding one of the others of the plurality ofsecond waveguides.
 15. The coupled system of claim 1, wherein the firstwaveguide and the at least one second waveguide are formed in a silicon(Si) photonic integrated circuit (PIC), the coupled system furthercomprising a semiconductor chip wafer bonded to the Si PIC, wherein: thesemiconductor chip comprises an active optical device optically coupledto an end of the first waveguide opposite the tapered end of the firstwaveguide; and the active optical device comprises an indium phosphide(InP)-based gain element or an InP-based pin detector.
 16. The coupledsystem of claim 1, further comprising a silicon (Si) photonic integratedcircuit (PIC), including: a substrate; silicon dioxide (SiO₂) box formedabove the substrate; a first layer formed above the SiO₂ box and inwhich the at least one second waveguide is formed as at least onesilicon nitride (SiN) waveguide; a second layer formed above the SiO₂box and above or below the first layer and in which the first waveguideis formed as a Si waveguide; and a plurality of dielectric layers formedabove the first layer, wherein: the Si PIC defines an etched windowthrough the plurality of dielectric layers down to the first layer,wherein the tapered end of the at least one second waveguide is exposedin the etched window; the etched window is laterally bounded on threesides by the dielectric layers of the Si PIC that are above the firstlayer; a top layer included in the dielectric layers of the Si PICincludes a plurality of metal dummies at least in a region of the toplayer that bounds the etched window.
 17. The coupled system of claim 16,wherein: the interposer comprises an interposer substrate, an interposercladding, and an interposer core, the interposer core and the interposercladding collectively forming the third waveguide; at least a bottomsurface of the interposer core is exposed and free of the interposercladding along the coupler portion of the third waveguide that ispositioned within the etched window above the tapered end of the atleast one second waveguide; the interposer further comprises at leastone interposer alignment ridge that extends downward from the interposercladding and is laterally offset from the third waveguide; the Si PICdefines at least one anchor window laterally offset from the etchedwindow; and the at least one interposer alignment ridge is positionedwithin the at least one anchor window to laterally align the thirdwaveguide with the one of the at least one second waveguide.
 18. Thecoupled system of claim 16, wherein: the interposer comprises a highindex glass waveguide block; the third waveguide comprises a high indexglass waveguide formed in the high index glass waveguide block; the highindex glass waveguide block defines one or more holes or grooveslaterally offset from the high index glass waveguide; the one or moreholes or grooves extend vertically upward from a bottom surface of thehigh index glass waveguide block; the coupled system further comprisesan epoxy underfill positioned within the etched window between at leasta portion of the first layer within the etched window and at least aportion of the bottom surface of the high index glass waveguide block; athickness of the epoxy underfill between the bottom surface of the highindex glass waveguide block and the first layer is less than 1micrometer; and each of the one or more holes or grooves is filled withthe epoxy underfill.
 19. The coupled system of claim 16, the interposercomprises a high index glass waveguide block; the third waveguidecomprises a high index glass waveguide formed in the high index glasswaveguide block; the high index glass waveguide block defines a v-groovelongitudinally adjacent to the high index glass waveguide; and thev-groove is configured to receive therein at least a portion of anoptical fiber and to optically align the optical fiber with the highindex glass waveguide.
 20. The coupled system of claim 1, wherein theinterposer comprises: a silicon dioxide (SiO₂) substrate; a siliconoxynitride (SiON) cladding coupled to the SiO₂ substrate; and a SiONcore surrounded on all sides within a first portion of the interposerand having at least one side exposed and free of SiON cladding within asecond portion of the interposer, wherein the SiON cladding and the SiONcore collectively form the third waveguide.
 21. The coupled system ofclaim 1, wherein the interposer comprises a polymer on glass interposer,including: a glass substrate; a polymer cladding coupled to the glasssubstrate; and a polymer core surrounded on all sides by the polymercladding within a first portion of the interposer and having at leastone side exposed and free of polymer cladding within a second portion ofthe interposer, wherein the polymer cladding and the polymer corecollectively form the third waveguide.
 22. The coupled system of claim1, further comprising a silicon (Si) photonic integrated circuit (PIC),including: a substrate; silicon dioxide (SiO₂) box formed above thesubstrate; a first layer formed above the SiO₂ box and in which the atleast one second waveguide and a plurality of second waveguides isformed; a second layer formed above the SiO₂ box and above or below thefirst layer and in which the first waveguide is formed as a Siwaveguide; and a SiN wavelength division demultiplexer (WDM demux)formed in the first layer, wherein an input of the SiN WDM demux iscoupled to one of the plurality of SiN waveguides and each of multipleoutputs of the SiN WDM demux is coupled to a corresponding different oneof the plurality of SiN waveguides.
 23. The coupled system of claim 22,further comprising a Si PIC polarization splitter formed in the Si PIC,wherein the Si PIC polarization splitter includes: a first SiN waveguideof the plurality of SiN waveguides formed in the first layer of the SiPIC, wherein the first SiN waveguide includes a coupler portion at afirst end of the first SiN waveguide; a second SiN waveguide of theplurality of SiN waveguides formed in the first layer of the Si PIC andspaced apart from the first SiN waveguide, wherein the second SiNwaveguide includes a coupler portion at a first end of the second SiNwaveguide; and the Si waveguide formed in the second layer of the SiPIC, wherein: the tapered end of the first waveguide includes a firsttapered end of the Si waveguide and the Si waveguide further includes asecond tapered end opposite the first tapered end; the first tapered endof the Si waveguide is aligned in two orthogonal directions with thecoupler portion of the first SiN waveguide such that the first taperedend of the Si waveguide overlaps in the two orthogonal directions and isparallel to the coupler portion of the first SiN waveguide; the twoorthogonal directions correspond to a length direction and a widthdirection of the first and second SiN waveguides; the second tapered endof the Si waveguide is aligned in the two orthogonal directions with thecoupler portion of the second SiN waveguide such that the second taperedend of the Si waveguide overlaps in the two orthogonal directions and isparallel to the coupler portion of the second SiN waveguide; each of thefirst and second tapered ends of the Si waveguide is configured toadiabatically couple most TE polarization of an input beam between acorresponding one of the first and second tapered ends and acorresponding one of the first and second SiN waveguides and to preventmost TM polarization of the input beam from being adiabatically coupledbetween the corresponding one of the first and second tapered ends andthe corresponding one of the first and second SiN waveguides; the SiNWDM demux comprises a first SiN WDM demux with its input opticallycoupled to a second end of the first SiN waveguide that is opposite thefirst end of the first SiN waveguide; and the coupled system furthercomprises a second SiN WDM demux formed in the first layer of the Si PICwith a plurality of outputs and an input optically coupled to a secondend of the second SiN waveguide that is opposite the first end of thesecond SiN waveguide.
 24. The photonic system of claim 23, furthercomprising a polarization rotator disposed in an optical path betweenthe second end of the second SiN waveguide and the input of the secondSiN WDM demux.